What is Equipment Evaluation? 

Equipment evaluation, equipment sizing or device withstand rating verification is based on short circuit results. It verifies if short circuit rating of electrical equipment is high enough comparing to calculated short circuit current. Such calculations should be normally included in design phase however for existing installation it is recommended to verify it, especially when there is no previous design report available. 

Why is it important?

Proper sizing of equipment is crucial for safe operation under normal and faulty conditions. Cables, switchgears, breakers, transformers are design to withstand specific short circuit current in time. For proper operation of the breaker, it is critical that the breaker can withstand specific currents. If it is undersized, it can lead be damaged and pose threat to people and rest of electrical installation. 

How this relates to Arc Flash? 

It is included as a recommended step in IEEE 1584. Besides this, it is important to know that our equipment withstand rating is related with performance under fault conditions and normal operation. Constant overload or dynamic stress can cause damage to it and affect proper operation. This can lead to nuisance tripping, no tripping, thermal damage or fire etc. We can select worst case scenario out of multiple scenarios, however for experienced engineers it is easy to identify it. 

Example: 
We have low voltage switchgear rated 2000A 50kA/1s 105kA peak. If we do not know peak value, it is good to check in IEC 61439 table 7 for details.

We calculated two values in two scenarios: 

1. Ik” ≈Ith=46,45kA and Ip=100,5kA it is at the end of rating but can be accepted. In practice, it is good to keep 5-10% safety margin. 

2. Ik” ≈Ith=53,38kA and Ip=119,44kA – this switchgear rating is too low, and we need to look for higher short circuit rating or verify short circuit study input data first.  

When comparing short circuit results with different equipment, IEC 60909 comes with help and indicate which values to compare: 

Summary of equipment evaluation using SKM Power Tools

Below is an example of summary of equipment evaluation for switchgears and panel boards made with use of SKM Power Tools in this case. It can look different for other software’s like ETAP, CYME, EasyPower, DigSilent however core values should be similar. Such verification gives a very useful overview and directly indicates possible rating issues.  

In specific cases it is necessary to verify X/R or R/X of network and network time constant if not known as it affects results. Especially when local generation exists on site, This is very visible in marine and offshore installations. We need to keep in mind also renewable sources, battery storage systems, (DR) UPS-es with bypass in our calculations.