IntroductionAs far as safety is concern, an electrical engineer need not only consider short circuit analysis in his design. Aside from short circuit calculation he should know to coordinate different elements in the electrical system. There are two basic coordination study involve in electrical design, viz:
- Coordination of over-current protection device (OCPD) and cable
- Upstream OCPD's and downstream OCPD's (selective coordination)
Coordination of Over-current Protection Device and CableWhy we need to coordinate the sytem?
- We need to coordinate OCPD and cable to ensure that during fault condition the circuit breaker or fuse will clear the system before the conductor start burning that would trigger an arc flash or fire.
- Coordination study is very critical since the interval of the clearing the fault and the start of burning of the cable is just a matter of second or split of a second.
To coordinate OCPD and Cable we need to understand first how to read the circuit breaker's time-current curve and the cable's damage curve. This two curves are given by the manufacturer, therefore during the design it is important to always consult with the manufacturers manual for this study.
|Fig. 1. Fuse Time-Current Curve|
|Fig. 2. Cable Damage Curve|
How to Coordinate OCPD and cable?
- Get the corresponding OCPD time-current curve and the cable damage curve.
- By the aid of a computer software or by manual process make the graphical scale of these graphs identical.
- When they are already in the same scale, look at the cable damage curve if it is in the right side of the OCPD's time-current curve and does not overlap. If it is then they are coordinated
|Fig. 3. Coordinated OCPD and Cable|
Figure 3 is an example of coordinated OCPD and cable. The red line represents cable damage curve and it shows that a short circuit current of 40,000 ampere trips the breaker after 2 cycles.
- 2 cycles at 60 Hz = ( 2 x 1/60 ) = 0.033 seconds
- 3 cycles at 60 Hz = ( 3 x 1/60 ) = 0.05 seconds
Note that the interval of the circuit breaker opening and the deterioration of the cable is only one cycle or 0.017 seconds (0.05 seconds - 0.033 seconds).
Therefore, it is not only important to test the integrity of the circuit breakers but also we need to perform coordination study to ensure that the cable damage curve should not intersect with circuit breaker curve.