Relay Co-ordination Approach

[Charz]

In a Project, We have our scope starting from the MCC Panel to the final loads. The upstream of MCC is by the Owner. We have to do the relay coordination for our MCC and had done it manually the coordination between the Main breaker and the outgoing feeders in the MCC supplied by us.


Now the owner says our relay co-ordination is wrong as per their coordination, since the setting in the upstream panels exceed the transformer damage curve.


My question is, What is the correct approach to do this co-ordination? The co-ordination should be done from downstream to upstream or from upstream to downstream?

 

[wbdvt]

Not sure about your use of the term relay for the coordination. Are there actual relays you are trying coordinate? If also including the molded case circuit breakers, breakers with electronic trip units and possibly fuses, you are doing a protective device coordination study.

Typically, one starts at the downstream end and works up. There may be iterations of going back down and then up to coordinate. In some cases absolute selectivity cannot be achieved. Obviously, software makes this easier to do.

Another aspect of coordination that is overlook is the effect on the incident energy (arc flash) on a piece of equipment. Sometimes ideal coordination results in very high incident energy values whereas a little mis-coordination could result in very low incident energy values.

 

[ron]

As long as your protection takes care of the transformer, then who cares if the upstream exceeds the damage curve. You have it taken care of down stream.

BTW, I start one protective device above my scope and work downstream when I only have a partial coordination project. That way I can see if I'm selective with upstream and take care of the protection that is within my scope, such as transformer protection if it is in my part of the path.