What is the Advantage of IDMT in Protective Relaying?

 

In protective relaying there are two philosophies available to effectively achieve selectivity and coordination by time grading two philosophies are available, namely: 

1. Definite time lag (DTL), or 
2. Inverse definite minimum time (IDMT). 

Traditionally, design engineers have regarded medium- and low-voltage networks to be of lower importance from a protection view, requiring only the so-called simpler type of IDMT overcurrent and earth fault relays on every circuit. In many instances, current transformer ratios were chosen mainly based on load requirements, whilst relay settings were invariably left to the commissioning engineer to determine. Most of the times, the relay settings had been chosen considering the downstream load being protected without an effort to coordinate with the upstream relays. 

Read: Basic Guide in Power System Protection

However, experience has shown that there has been a total lack of appreciation of the fundamentals applicable to these devices. Numerous incidents have been reported where breakers have tripped in an uncoordinated manner leading to extensive network disruption causing longer down times or failed to trip causing excessive damage, extended restoration time and in some cases loss of life. 

In Definite Time Lag or DTL, the relays are graded using a definite time interval of approximately 0.5 s. The relay R3 at the extremity of the network is set to operate in the fastest possible time, whilst its upstream relay R2 is set 0.5 s higher. Relay operating times increase sequentially at 0.5 s intervals on each section moving back towards the source as shown below,

Definite Time Lag or DTL

The problem with this philosophy is, the closer the fault to the source the higher the fault current, the slower the clearing time – exactly the opposite to what we should be trying to achieve. 

Read: IEC 61850 Logical Nodes and Data Classes in Power System Automation Data Modelling

On the other hand, inverse curves as shown in the figure below which describes operating faster at higher fault currents and slower at the lower fault currents, thereby offering us the features that we desire. This explains why the IDMT philosophy has become standard practice throughout many countries over the years

Although not appreciated by many engineers, the widespread use of inverse definite minimum time overcurrent and earth fault (IDMT OCEF) relays as the virtual sole protection on medium- and low-voltage networks requires as much detailed study and applications knowledge as does the more sophisticated protection systems used on higher voltage networks.  

Applying IDMT in the System

When deciding to apply IDMT relays to a network, a number of important points have to be considered. Firstly, it must be appreciated that IDMT relays cannot be considered in isolation. They have to be set to coordinate with both upstream and downstream relays. 

Their very purpose and being is to form part of an integrated whole system. Therefore, whoever specifies this type of relay should also provide the settings and coordination curves as part of the design package to show that he knew what he was doing when selecting their use. This very important task should not be left to others and once set, the settings must not be tampered (even by the operating staff ) as otherwise coordination is lost.

Minimum Grading Intervals

To engineers planning the protection for a medium- to low-voltage network and wishing to adopt the widespread use of the IDMT OCEF relay, the above can be summarized as follows: 

• Design networks with a minimum number of grading levels possible. 
• Choose CT ratios based on fault current – not load current. 
• Consider using 1 A secondary. 
• Check CT magnetization curves for knee-point voltage and internal resistance. 
• Connect ammeters, etc. onto own metering cores. 
• Provide relay settings and coordination curves as part of the design package. 
• Be careful when choosing relay plug tap setting on electromechanical relays. The lower the tap, the higher the burden. 
• Relays should not pick-up for healthy conditions such as permissible transient overloads, starting surges and reconnection of loads, which have remained connected after a prolonged outage. 
• Care should also be taken that the redistribution of load current after tripping does not cause relays on healthy circuits to pick-up and trip. 
• HV IDMT relays on transformers should trip both HV and LV breakers. 
• Normal inverse curves should not be selected for overload protection. Rather use the inverse characteristic for this duty. 
• Take advantage of the additional features offered by the modern electronic relays, e.g. fixed very low burden, integral high-set, breaker fail and busbar blocking protections, event memory, etc. However, remember, one has to do the same calculation exercises for settings and draw coordination curves whether the relays are of the electronic or electromechanical design. 
• Finally, if the switchgear suppliers also manufacture relays, do not expect them to do the protection application settings free of charge as part of the service. If this is required, specify this as a separate cost item in the specification. 

Many problems down the line can be avoided and the performance, efficiency and safety of the plant improved if a protection engineer is included in the design team, if not full time, but at least to do an audit on the proposals. Finally, remember – while  IDMT relays are the most well known and the cheapest, they are in fact the most difficult relays to set. 

Reference: 

Practical Power System Protection | Download

Authors: 

• Les Hewitson
• Mark Brown PrEng, DipEE, BSc (ElecEng), Senior Staff Engineer, IDC Technologies, Perth, Australia  
• Ben Ramesh Ramesh and Associates, Perth, Australia

Series editor: 

• Steve Mackay FIE(Aust), CPEng, BSc (ElecEng), BSc (Hons), MBA, Gov. Cert. Comp., Technical Director – IDC Technologies.

How to Interpret the Different Region of Circuit Breaker Curves

 

Circuit Breaker
by: EATON 

Circuit breaker time-current characteristic curves are read similar to fuse curves. The horizontal axis represents the current, and the vertical axis represents the time at which the breaker interrupts the circuit. When using molded case circuit breakers of this type, there are four basic curve considerations that must be understood. 

These are:

1. Overload Region
2. Instantaneous Region

Overload Region

The opening of a molded case circuit breaker in the overload region is generally accomplished by a thermal element, while a magnetic coil is generally used on power breakers. Electronic sensing breakers will utilize CTs. As can be seen, the overload region has a wide tolerance band, which means the breaker should open within that area for a particular overload current.

Instantaneous Region

The instantaneous trip (I.T.) setting indicates the multiple of the full load rating at which the circuit breaker will open as quickly as possible. The instantaneous region is represented in the following curve and is shown to be adjustable from 5x to 10x the breaker rating. When the breaker coil senses an overcurrent in the instantaneous region, it releases the latch which holds the contacts closed.

Interrupting Rating

The interrupting rating of a circuit breaker is a critical factor concerning protection and safety. The interrupting rating of a circuit breaker is the maximum fault current the breaker has been tested to interrupt in accordance with testing laboratory standards. Fault currents in excess of the interrupting rating can result in the destruction of the breaker and equipment and possible injury to personnel. In other words, when the fault level exceeds the circuit breaker interrupting rating, the circuit breaker is no longer a protective device.

Read: What are the different types of circuit breakers and their uses?

Illustration: Medium to High-Level Fault Currents–Circuit Breakers

The following curve illustrates a 400A circuit breaker ahead of a 90A breaker. Any fault above 1500A on the load side of the 90A breaker will open both breakers. 

The 90A breaker will generally unlatch before the 400A breaker. However, before the 90A breaker can separate its contacts and clear the fault current, the 400A breaker has unlatched and also will open. Assume a 4000A short circuit exists on the load side of the 90A circuit breaker. 

Read: What is the advantage of using Thermal Magnetic Circuit Breaker in motor control?

The sequence of events would be as follows: 

1. The 90A breaker will unlatch (Point A) and free the breaker mechanism to start the actual opening process.
2. The 400A breaker will unlatch (Point B) and it, too, would begin the opening process. Once a breaker unlatches, it will open. At the unlatching point, the process is irreversible. 
3. At Point C, the 90A breaker will have completely interrupted the fault current. 
4. At Point D, the 400A breaker also will have completely opened the circuit. Consequently, this is a non-selective system, causing a complete blackout to the other loads protected by the 400A breaker.

Reference: 

Cooper Bussman |  Download

Why is Voltage Rating Important in Selecting Circuit Protection Devices?

 

This is an extremely important rating for overcurrent protective devices (OCPDs). The proper application of an overcurrent protective device according to its voltage rating requires that the voltage rating of the device be equal to or greater than the system voltage. When an overcurrent protective device is applied beyond its voltage rating, there may not be any initial indicators. Adverse consequences typically result when an improperly voltage rated device attempts to interrupt an overcurrent, at which point it may self-destruct in an unsafe manner. 

There are two types of OCPD voltage ratings: 

1. straight voltage rated and;
2.  slash voltage rated. 

The proper application is straightforward for overcurrent protective devices with a straight voltage rating (i.e.: 600V, 480V, 240V) which have been evaluated for proper performance with full phase-to-phase voltage used during the testing, listing and marking. For instance, all fuses are straight voltage rated and there is no need to be concerned about slash ratings. 

However, some mechanical overcurrent protective devices are slash voltage rated (i.e.: 480/277, 240/120, 600/347). Slash voltage rated devices are limited in their applications and extra evaluation is required when they are being considered for use. The next section covers fuse voltage ratings followed by a section on slash voltage ratings for other type devices.

Overcurrents 

An overcurrent is either an overload current or a short-circuit current. The overload current is an excessive current relative to normal operating current, but one which is confined to the normal conductive paths provided by the conductors and other components and loads of the distribution system. As the name implies, a short-circuit current is one which flows outside the normal conducting paths.

Overload

Overloads are most often between one and six times the normal current level. Usually, they are caused by harmless temporary surge currents that occur when motors start up or transformers are energized. Such overload currents, or transients, are normal occurrences. Since they are of brief duration, any temperature rise is trivial and has no harmful effect on the circuit components. (It is important that protective devices do not react to them.) Continuous overloads can result from defective motors (such as worn motor bearings), overloaded equipment, or too many loads on one circuit. 

Such sustained overloads are destructive and must be cut off by protective devices before they damage the distribution system or system loads. However, since they are of relatively low magnitude compared to short-circuit currents, removal of the overload current within a few seconds to many minutes will generally prevent equipment damage. A sustained overload current results in overheating of conductors and other components and will cause deterioration of insulation, which may eventually result in severe damage and short circuits if not interrupted.

Short Circuits 

Whereas overload currents occur at rather modest levels, the short-circuit or fault current can be many hundred times larger than the normal operating current. A high level fault may be 50,000A (or larger). If not cut off within a matter of a few thousandths of a second, damage and destruction can become rampant–there can be severe insulation damage, melting of conductors, vaporization of metal, ionization of gases, arcing, and fires. Simultaneously, high level short-circuit currents can develop huge magnetic field stresses. The magnetic forces between bus bars and other conductors can be many hundreds of pounds per linear foot; even heavy bracing may not be adequate to keep them from being warped or distorted beyond repair.

Fuse Protection

The fuse is a reliable overcurrent protective device. A “fusible” link or links encapsulated in a tube and connected to contact terminals comprise the fundamental elements of the basic fuse. Electrical resistance of the link is so low that it simply acts as a conductor. However, when destructive currents occur, the link very quickly melts and opens the circuit to protect conductors and other circuit components and loads. 

Modern fuses have stable characteristics. Fuses do not require periodic maintenance or testing. Fuses have three unique performance characteristics: 

1. Modern fuses have an extremely “high interrupting” rating–can open very high fault currents without rupturing.
2. Properly applied, fuses prevent “blackouts.” Only the fuse nearest a fault opens without upstream fuses (feeders or mains) being affected–fuses thus provide “selective coordination.” (These terms are precisely defined in subsequent pages.)
3. Fuses provide optimum component protection by keeping fault currents to a low value…They are said to be “current- limiting.”

Download the Full Document Here

Reference: 

• Cooper Bussman

Electric Motor Protection in Case of Voltage Unbalance and Single Phasing

 

Voltage Unbalance

When the voltage between all three phases is not equal, the current values in each phase will also become unbalanced. According to NEMA, the maximum voltage unbalance is limited only to 1% both for electric motors and generators. When the voltage unbalance occurs, the current increases gradually in the motor winding which if it continues, the motor will be damaged. Therefore it is necessary derate the motor according to the expected voltage unbalance. 

If in case, the derating is not possible and the voltage unbalance still persists, the loading in this case must be reduced accordingly. This method must be considered to avoid the damage of the equipment.

Causes of Unbalanced Voltage 

• Connecting unequal single-phase loads. This is why many consulting engineers specify that loading of panelboards be balanced to ± 10% between all three phases.
• Open delta connections. 
• Transformer connections open - causing a single-phase condition. 
• Improper tap settings on transformer banks.  
• Transformer impedances (Z) of single-phase transformers connected into a “bank” not the same. 
• The capacitors used in power factor correction capacitors are not the same or some of them are off the line

Insulation Life The effect of voltage unbalance on the insulation life of a typical T-frame motor having Class B insulation, running in a 40°C ambient, loaded to 100%, is as follows:

Note that motors with a service factor of 1.0 do not have as much heat withstand capability as do motors having a service factor of 1.15. Older, larger U-frame motors, because of their ability to dissipate heat, could withstand overload conditions for longer periods of time than the newer, smaller T-frame motors.

Insulation Classes 

The following shows the maximum operating temperatures for different classes of insulation. 

• Class A Insulation = 105°C 
• Class B Insulation = 130°C 
• Class F Insulation = 155°C 
• Class H Insulation = 180°C

Reference: 

• Cooper Bussman

What is the Difference Between Interrupting Rating and Interrupting Capacity?

 

Main Distribution Panel

Many electrical engineers thought that interrupting rating and interrupting capacity has a similar meaning. For this reason, we need to know its key differences and its effect when applied to electrical design or installations. 

Interrupting Rating

Interrupting rating is the maximum short-circuit current that an overcurrent protective device can safely interrupt under standard test conditions. One should notice the term “under standard test conditions” which means, it is important to determine how the overcurrent protective device is tested in order to assure it is properly applied. 

Interrupting Capacity

According to the IEEE Standard Dictionary of Electrical and Electronic Terms, interrupting capacity is the current at rated voltage that the device can interrupt. 

Standard Test Conditions - Fuses

The branch circuit fuses are tested without any additional conductor in the test circuit. For example, if a fuse has an interrupting rating of 300 kA, the test circuit is calibrated to have at least 300 kA at the rated fuse voltage. During the test circuit calibration, a bus bar is used in place of the fuse to check the proper short-circuit current. Then the bus bar is removed and the fuse is inserted; the test is then conducted. If the fuse passes the test, the fuse is marked with this interrupting rating (300 kA). 

In the procedures just outlined for fuses, there are no extra conductors inserted into the test circuit after the short-circuit current is calibrated. A major point is that the fuse interrupts an available short-circuit current at least equal to or greater than its marked interrupting rating. 

In other words, because of the way fuses are short-circuit tested (without additional conductor impedance), their interrupting capacity is equal to or greater than their marked interrupting rating.

Standard” Test Conditions - Circuit Breakers

Compare to fuses, it is not the case with circuit breakers. This is because of the way circuit breakers are short circuit tested (with additional conductor impedance), their interrupting capacity can be less than their interrupting rating. 

When the test circuit is calibrated for the circuit breaker interrupting rating tests, the circuit breaker is not in the circuit. After the test circuit has been verified to the proper level of short-circuit current, the circuit breaker is placed into the circuit. Accordingly, in addition to the circuit breaker, important lengths of conductors are permitted to be added to the circuit after the calibration. This additional conductor impedance can result in a significantly lower short-circuit current. 

So a circuit breaker marked with an interrupting rating of 22,000A may in fact have an interrupting capacity of only 9,900A.

Reference: 

• Cooper Bussman.

Download the full article here.

What are the Characteristics of Fuse as Circuit Protection?

Fuse

 

A fuse is the simplest device for interrupting a circuit experiencing an overload or a short circuit. A typical fuse, like the one shown below, consists of an element electrically connected to end blades or ferrules. The element provides a current path through the fuse. The element is enclosed in a tube and surrounded by a filler material. 

The National Electrical Code® defines overcurrent as any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault (Article 100-definitions). Circuit protection would be unnecessary if overloads and short circuits could be eliminated. Unfortunately, overloads and short circuits do occur. To protect a circuit against these currents, a protective device must determine when a fault condition develops and automatically disconnect the electrical equipment from the voltage source.

Nontime-delay Fuses

Nontime-delay fuses provide excellent short circuit protection. Short-term overloads, such as motor starting current, may cause nuisance openings of nontime-delay fuses. They are best used in circuits not subject to large transient surge currents. Nontime-delay fuses usually hold 500% of their rating for approximately one-fourth second, after which the current carrying element melts. This means that these fuses should not be used in motor circuits which often have inrush (starting) currents greater than 500%.

Time Delay Fuses

Time-delay fuses provide overload and short circuit protection. Time-delay fuses usually allow five times the rated current for up to ten seconds. This is normally sufficient time to allow a motor to start without nuisance opening of the fuse unless an overload persists.

Read: How to Select Overcurrent Protection Devices?

Fuse Ratings

Fuses have a specific ampere rating, which is the continuous current carrying capability of a fuse. The ampere rating of a fuse, in general, should not exceed the current carrying capacity of the circuit. For example, if a conductor is rated for 10 amperes, the largest fuse that would be selected is 10 amperes. There are some specific circumstances when the ampere rating is permitted to be greater than the current carrying capacity of the circuit. For example, motor and welder circuits can exceed conductor ampacity to allow for inrush currents and duty cycles within limits established by the NEC.

The voltage rating of a fuse must be at least equal to the circuit voltage. The voltage rating of a fuse can be higher than the circuit voltage, but never lower. A 600 volt fuse, for example, can be used in a 480 volt circuit. A 250 volt fuse could not be used in a 480 volt circuit.

Fuses are also rated according to the level of fault current that they can interrupt. This is referred to as ampere interrupting capacity (AIC). When applying a fuse, one must be selected which can sustain the largest potential short circuit current which can occur in the selected application. The fuse could rupture, causing extensive damage, if the fault current exceeds the fuse interrupting rating.

UL Fuse Classification

Fuses are grouped into current limiting and non-current limiting classes based on their operating and construction characteristics. Fuses that incorporate features or dimensions for the rejection of another fuse of the same ampere rating but with a lower interruption rating are considered current-limiting fuses. Underwriters Laboratories (UL) establishes and standardizes basic performance and physical specifications to develop its safety test procedures. These standards have resulted in distinct classes of low voltage fuses rated at 600 volts or less. The following chart lists various UL fuse classes.

UL Fuse Classification

Reference: 

• Siements

How to Select Overcurrent Protection Devices?

Circuit Protection Device

It is a standard rule that all electrical installations must be protected against overcurrent or short circuit by means of devices that will operate automatically to prevent injury to persons and livestock and damage to the installation, including the cables. As such, the overcurrent devices must be of adequate breaking capacity and be so constructed that they will interrupt the supply without danger. Also, the cables must be able to carry these overcurrents without damage. 

Fault currents

Fault currents arise as a result of a fault in the cables or the equipment. There is a sudden increase in current, perhaps 1O or 20 times the cable rating, the current being limited by the impedance of the supply, the impedance of the cables, the impedance of the fault and the impedance of the return path. The current should be of short duration, as the overcurrent device should operate.

Overload currents

Overload currents do not arise as a result of a fault in the cable or equipment. They arise because the current has been increased by the addition of further load. Overload protection is only required if overloading is possible. It would not be required for a circuit supplying a fixed load, although fault protection would be required.

For example, a circuit load supplying a 7.2 kW shower will not increase unless the shower is replaced, when the adequacy of the circuit must be checked against the new load criteria. A distribution circuit supplying a number of buildings could be overloaded by additional machinery being installed in one of the buildings supplied. 

Overload currents are likely to be of the order of 1 .5 to 2 times the rating of the cable, whereas fault currents may be of the order of 10 to 20 times  the rating. Overloads of less than 1 .2 to 1 .6 times the device rating are unlikely to result in operation of the device. 

British Standard 7671 and IEC 60364 requires that every circuit be designed so that small overloads of long duration are unlikely to occur. It is usual for one device in the circuit to provide both fault protection and overload protection. A common exception is the overcurrent devices to motor circuits, where the overcurrent device at the origin of the circuit provides protection against fault currents and the motor starter will be providing protection against overload.

Read: How to Select Proper Type of Miniature Circuit Breakers MCBs

Selecting protective devices

The type of protective device chosen will depend on a number of factors, including:

• the nature or type of load
• the prospective fault current P1 at that point of the installation
• any existing equipment
• the user of the installation, as a CB is easier to reset than a bolted­ type HRC fuse.

Breaking capacity

There is a limit to the maximum current that an overcurrent protective device (fuse or circuit breaker) can interrupt. This is called the rated short-circuit capacity or breaking capacity. BS 7671 and IEC 60364 requires the prospective fault current under both short-circuit and earth-fault conditions to be determined at every relevant point of the complete installation. This means that at every point where switchgear is installed, the maximum fault current must be determined to ensure that the switchgear is adequately rated to interrupt the fault currents.

Circuit breakers  have two short-circuit capacity  ratings. 

• Ics = is the value of fault current up to which the device can operate safely and remain suitable and serviceable after the fault. 
• Icn = is the value above which the device would not be able to interrupt faults safely . This could lead to the danger of explosion during faults of this magnitude or, even worse, the contacts welding and not interrupting the fault.

Any faults that occur between these two ratings will be interrupted safely but the device will probably require replacement.

circuit breaker, selection of protective devices, fuses, shot circuit, overcurrent, over-current, show to select overcurrent protection devices.

What are the IEEE/ANSI Device Numbers Used in Power System Protection ?

Sample Application in Generator Protection 
(Photo Credit: General Electic)

In power system, the protection and control of equipment is represented by ANSI device numbers, with corresponding suffix letters when necessary, in relation to the functions they perform. The numbers are based on a system that is adopted by a standard for automatic switchgear by Institute of Electrical and Electronics Engineers (IEEE), and incorporated in American Standard C37.2-1996. 

This system is used  along with the diagrams that can be found in instruction books and in specifications. The International Electrotechnical Commission (IEC) standards 617 and 60617 also provide different symbols and terminology for most of the device numbers that are defined by C37.2. 

The second portion of this document provides a brief overview of a few of the more common IEC symbols used.

Read: Related Articles in Fault Analysis

1 - Master Element

2 - Time Delay Starting or Closing Relay

3 -  Checking or Interlocking Relay

4 - Master Contactor

5 - Stopping Device

6 - Starting Circuit Breaker

7 - Rate of Change Relay

8 - Control Power Disconnecting Device

9 - Reversing Device

10 - Unit Sequence Switch

11 - Multifunction Device

12 - Overspeed Device/Protection

13 - Synchronous-Speed Device

14 - Under-speed Device

15 - Speed or Frequency Matching Device

16 - Communication Networking Device

17 - Shunting or Discharge Switch

18 - Accelerating or Decelerating Device

19 - Motor Starter / Starting-to-Running Transition Contactor

20 - Electrically-Operated Valve

21 - Distance Relay

21G - Ground Distance

21P -  Phase Distance

22 - Equalizer Circuit Breaker

23 - Temperature Control Device

24 - Volts-per-Hertz Relay / Overfluxing

25 - Synchronizing or Synchronism-Check Device

26 - Apparatus Thermal Device

27 - Undervoltage Relay

27 TN  - Phase Undervoltage

27 X  - Third Harmonic Neutral Undervoltage

27 AUX - Undervoltage Auxiliary Input

27/ 27 X - Bus/Line Undervoltage

28 - Flame Detector

29 - Isolating Contactor

30 - Annunciator Relay

31 - Separate Excitation Device

32 - Directional Power Relay

32 L - Low Forward Power

32 N - Wattmetric Zero-Sequence Directiona

32 P - Directional Power

32 R - Reverse Power

33 - Position Switch

34 - Master Sequence Device

35 - Brush-Operating or Slip-ring Short Circuiting Device

36 - Polarity or Polarizing Voltage Device

37-  Undercurrent or Underpower Relay

37P - Underpower

38 -  Bearing Protective Device / Bearing Rtd

39 - Mechanical Condition Monitor 

40 - Field Relay / Loss of Excitation 

41 -  Field Circuit Breaker 

42 -  Running Circuit Breaker 

43 -  Manual Transfer or Selector Device 

44 - Unit Sequence Starting Relay

45 - Atmospheric Condition Monitor

46 -  Reverse-Phase or Phase Balance Current Relay or Stator Current Unbalance 

47 - Phase-Sequence or Phase Balance Voltage Relay

48 - Incomplete Sequence Relay / Blocked Rotor 

49 - Machine or Transformer Thermal Relay / Thermal Overload 

49 - RTD RTD Biased Thermal Overload 

Read: What are the Conditions in Selecting Current Transformer in Protective Relaying

50 - Instantaneous Overcurrent Relay 50BF Breaker Failure 

50 - DD Current Disturbance Detector 

50G - Ground Instantaneous Overcurrent 

50N -  Neutral Instantaneous Overcurrent 

50P - Phase Instantaneous Overcurrent 

50_2 -  Negative Sequence Instantaneous Overcurrent 

50/27 -  Accidental Energization 

50/74 - Ct Trouble 

50/87 - Instantaneous Differential 50EF End Fault Protection 

50IG - Isolated Ground Instantaneous Overcurrent 

50LR - Acceleration Time 

50NBF -  Neutral Instantaneous Breaker Failure 

50SG - Sensitive Ground Instantaneous Overcurrent 

50SP -  Split Phase Instantaneous Current 

51 -  Ac Time Overcurrent Relay

51 - Overload 

51G - Ground Time Overcurrent 

51N - Neutral Time Overcurrent 

51P - Phase Time Overcurrent 

51V - Voltage Restrained Time Overcurrent 

51R - Locked / Stalled Rotor

51_2 - Negative Sequence Time Overcurrent 

52 - Ac Circuit Breaker 

53 - Exciter or Dc Generator Relay 

54 - Turning Gear Engaging Device 

55 - Power Factor Relay 

56 - Field Application Relay 

57 - Short-Circuiting or Grounding Device 

58 - Rectification Failure Relay 

59 - Overvoltage Relay 

59B - Bank Phase Overvoltage 

59P - Phase Overvoltage 

59N - Neutral Overvoltage 

59NU - Neutral Voltage Unbalance 

59P - Phase Overvoltage 

59X - Auxiliary Overvoltage 

59_2 - Negative Sequence Overvoltage

60 - Voltage or Current Balance Relay 

60N - Neutral Current Unbalance 

60P - Phase Current Unbalance 

61 - Density Switch or Sensor 

62 - Time-Delay Stopping or Opening Relay 

63 - Pressure Switch Detector 

64 - Ground Protective Relay 

64F - Field Ground Protection 

64S - Sub-harmonic Stator Ground Protection 

64TN - 100% Stator Ground 

65 - Governor 

66 - Notching or Jogging Device/Maximum Starting Rate/Starts Per Hour/Time Between Starts 

67 - Ac Directional Overcurrent Relay 

67G - Ground Directional Overcurrent 67N Neutral Directional Overcurrent 

67P - Phase Directional Overcurrent 

67SG - Sensitive Ground Directional Overcurrent 

67_2 - Negative Sequence Directional Overcurrent 

68 - Blocking Relay / Power Swing Blocking 

69 - Permissive Control Device 

70 - Rheostat 

71 - Liquid Switch 

72 - Dc Circuit Breaker 

73 - Load-Resistor Contactor 

74 - Alarm Relay 

75 - Position Changing Mechanism

76 - Dc Overcurrent Relay 

77 - Telemetering Device 

78 -  Phase Angle Measuring or Out-of-Step Protective Relay 

78V - Loss of Mains 

79 - Ac Reclosing Relay / Auto Reclose 

80 - Liquid or Gas Flow Relay 

81 - Frequency Relay 

81O - Over Frequency 

81R - Rate-of-Change Frequency 

81U - Under Frequency 

82 - Dc Reclosing Relay 

83 - Automatic Selective Control or Transfer Relay 

84 - Operating Mechanism 

85 - Carrier or Pilot-Wire Receiver Relay 

86 - Locking-Out Relay 

87 - Differential Protective Relay 

87B - Bus Differential 

87G - Generator Differential 

87GT - Generator/Transformer Differential 

87LG - Ground Line Current Differential 

87S - Stator Differential 

87S - Percent Differential 

87L - Segregated Line Current Differential 

87M - Motor Differential 

87O - Overall Differential 

87PC - Phase Comparison 

87RGF - Restricted Ground Fault 

87T - Transformer Differential 

87V - Voltage Differential 

88 - Auxiliary Motor or Motor Generator 

89 - Line Switch 

90 - Regulating Device 

91 - Voltage Directional Relay 

92 - Voltage And Power Directional Relay 

93 - Field-Changing Contactor 

94 - Tripping or Trip-Free Relay 

50/74 - Ct Supervision 2

7/50 - Accidental Generator Energization 

27TN/59N - 100% Stator Earth Fault

Download the whole document here

What are the Different Techniques to Ensure Effective Grounding?

 

Grounding is important in an electrical system since it provide the lowest resistance path to the ground. Circuit protection device such as circuit breakers have maximum allowable resistance to operate properly. In case that the required value of resistance goes beyond the allowable value, it will not trip in due time. The right value of earth fault loop impedance is needed.

Read: What is Earth Fault Loop Impedance?

For example, the table below shows the maximum value of overall earth fault loop impedance in order to comply with BS 7671 and IEC 60364 to disconnection time. 

In general, the following are the purpose of effective grounding. 

1. To provide safety to personnel during normal and fault conditions by limiting step and touch potential.
2. To assure correct operation of electrical/electronic devices. 
3. To prevent damage to electrical/electronic apparatus. 
4. To dissipate lightning strokes. 
5. To stabilize voltage during transient conditions and to minimize the probability of flashover during transients. 
6. To divert stray RF energy from sensitive audio, video, control, and computer equipment.

Techniques to Ensure Effective Grounding

1. Use of Ground Electrode (single electrode)

In this article, we refer to ‘ground electrode‘ or ‘grounding system‘ to describe these different methods of grounding. It should be noted that there are many different types of grounding systems available. 

See: Earthing System According to BS 7671

The type installed will depend on the local conditions and the required function of the grounding system The simplest form of grounding element is the ground stake, this can take many forms with a variety of lengths from a few feet to many feet long made of materials such as brass, galvanized or stainless steel, the size and material as required locally The simple ground rod can be used for lightning protection on stand-alone structures such as pole mounted transformers or radio towers, it can also be used as a back up to a utility ground.

2. Multiple Grounding Electrode

A group of connected electrodes will have a more complicated interaction, typically configurations like this are present around substation sites and sensitive buildings. A slightly more complicated version of the rod system is the ground rod group, this is typically for lightning protection on larger structures or protection around potential hotspots such as substations.

This is typically for lightning protection on larger structures or protection around potential hotspots such as substations.

3. Ground Plates

This technique is used for areas where there is rock (or other poor conducting material) fairly close to the surface ground plates are preferred as they are more effective. Ground plates are used widely in telecoms applications.  They are particularly good where the deeper ground has high resistivity.

For areas where there is rock (or other poor conducting material) fairly close to the surface ground plates are preferred as they are more effective. 

4. Ground Mesh

A ground mesh consists of network of bars connected together, this system is often used at larger sites such as electrical substations. Ground meshes can be part of the foundations of structure.  At substations and generating site the metal parts of the foundations will all be bonded together and form part of the overall grounding systems.  At substation site an area of ground could be reserved at the start of the life of the substation with a ground mesh under the whole of the site.  As the site grows over a period of years new equipment can easily be installed and grounded by the mesh.  This ensures that the whole of the site remains at the same potential should a fault occur.

As a part of the design, we need to consider the soil resistivity so that we can determine the exact grounding techniques that we are going to use. Soil resistivity varies widely depending on soil type, from as low as 1 Ohmmeter for moist loamy topsoil to almost 10,000 Ohm-meters for surface limestone. Moisture content is one of the controlling factors in earth resistance because electrical conduction in soil is essentially electrolytic. 

How to Calculate Motor Circuit Branch Circuit Protection According to NEC 430.52

 

Motor Circuit Protection

Motor circuit protection describes the short-circuit protection of conductors supplying power to the motor, the motor controller, and motor control circuits/conductors. 430.52 provides the maximum sizes or settings for overcurrent devices protecting the motor branch circuit. A branch circuit is defined in Article 100 as “The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).”

NEC Motor Control Protection Requirements

Note that the branch circuit extends from the last branch circuit overcurrent device to the load. Table 430.52 lists the maximum sizes for Non-Time-Delay Fuses, Dual Element (Time-Delay) Fuses, Instantaneous Trip Circuit Breakers, and Inverse Time Circuit Breakers. Sizing is based on full load amp values shown in Table 430.247 through 430.250, not motor nameplate values. For example, the maximum time-delay fuse for a 10HP, 460 volt, 3 phase motor with a nameplate FLA of 13 amps would be based on 175% of 14 amps, not 175% of 13 amp.

Courtesy: Cooper Bussmann

 

Standard sizes for fuses and fixed trip circuit breakers, per 240.6, are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000 5000, and 6000 amps. Additional standard fuse sizes are 1, 3, 6, 10, and 601 amps. 

The exceptions in 430.52 allow the user to increase the size of the overcurrent device if the motor is not able to start. All Class CC fuses can be increased to 400%, along with non-time-delay fuses not exceeding 600 amps. Time-delay (dual-element) fuses can be increased to 225%. All Class L fuses can be increased to 300%. Inverse time (thermal-magnetic) circuit breakers can be increased to 400% (100 amp and less) or 300% (larger than 100 amps). Instant trip circuit breakers may be adjusted to 1300% for other than Design B motors and 1700% for energy efficient Design B motors. 

• 430.52(C)(2) reminds the user that the maximum device ratings which are shown in a manufacturer’s overload relay table must not be exceeded even if higher values are allowed by other parts of 430.52. 
• 430.52(C)(3) details the requirements that instant-trip CBs can only be used if part of a listed combination motor controller. 

Courtesy: Cooper Bussmann

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