How to Perform Coordination Study of OCPD and Cable in Electrical Design

Introduction

As 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 Cable

Why 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

Figure 1 is an example of an OCPD time-current curve where the protection characteristic of the fuse against overcurrent and short circuit is describe. Most of the circuit protection nowadays are inverse time delay (ITD), meaning to say the circuit breaker will open in a shortest possible time when the current is high. In the example given that circuit breaker will open at less than 0.002 second (minimum) and less than 0.03 second (maximum) when the current is 30,000 amperes.

Fig. 2. Cable Damage Curve

Figure 2 is an example of cable damage curve and it tells us that the deterioration of the current is shorter when the amount of current passing is high. In this example when approximately 800 amperes will flow in the cable it will break at 0.10 second.

How to Coordinate OCPD and cable?

1. Get the corresponding OCPD time-current curve and the cable damage curve. 
2. By the aid of a computer software or by manual process make the graphical scale of these graphs identical.
3. 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

Interpretation

Figure 3 is an example of coordinated OCPD and cable. The red line represents cable damage curve and it tells us that when there is a short circuit of 40,000 amperes the circuit breaker will trip at 2 cycles.

•  2 cycles at 60 Hz = ( 2 x 1/60 ) = 0.033 seconds

If the OCPD will not clear the fault and one more cycle will take place then the cable starts to burn,

• 3 cycles at 60 Hz = ( 3 x 1/60 ) = 0.05 seconds

During this time the cable start to burn and may cause fire or arc flash. Take note that the interval of the circuit breaker opening and the deterioration of the cable is only one cycle. Being a designer if we are not comfortable with that set-up then we are free to choose other options such as setting the 40k tripping time at 1 cycle and choose a cable that deteriorates time at 5 cycles for 40k amperes.