Coordination question about Low Voltage Power Circuit Breakers in a 480V Switchgear Instant Setting 2

[bdn2004]

Attached is out of the 1986 Buff Book in the Coordination Section example. It's a Main (green) and a Feeder Breaker (yellow)on a 480V switchgear and the 100A molded case circuit breaker on the very bottom (white).

I notice that the Feeder Breaker starts to trip at 0.1 seconds delay. In other words it's out of the instantaneous range. To me this makes sense, as the small MCCB out on the floor will have a chance to act. Is this still logical in 2020 ? Or has arc-flash made this method obsolete?

I'm looking at an existing study and all of the feeder breakers have instantaneous settings that will all conflict with all downstream devices - and the whole circuit will be shut down as a result of a short circuit.

 

[mparenteau]

Have you checked to see if your CBs can be selectively coordinated (even though overlapping instantaneous regions)? If not, I don't believe the Buff book is out of date necessarily...there's now more "give and take" analysis with regards to coordination and arc flash mitigation.

Mike

 

[dpc]

At the risk of sounding like a broken record - it's ALWAYS a compromise. If you put instantaneous trip on every breaker and set them all a minimum, the arc-flash energy will generally be quite low. If you want some level of coordination, then you will generally have to accept some reduction in protection and increase in incident energy. Things like zone selective interlocking can help, but for the most part, these systems are coordinated based on time. If the goal is to achieve the lowest possible incident energy, that will generally result in less than optimal coordination.

I don't think selectivity has gone out of fashion, but some have become so concerned with arc-flash that they are willing to sacrifice coordination to reduce the incident energy.

Maintenance mode settings are another approach, but obviously coordination is sacrificed when the maintenance mode is active.

Regarding the TCC - you can improve the situation by setting the short-time delay as low as practical. Arc-flash wasn't a consideration in the Buff Book. Also, in the US, the short-circuit rating for motor control centers is based on a three-cycle fault unless specified for something longer. Tripping the feeder breaker to the MCC on short-time trip instead of IT would result in a fault duration that exceeds three cycles. But it was commonly done to improve coordination.

Cheers,

Dave

 

[bdn2004]

It guess the logic is ... the wires come into the downstream device, right there at that point before it is terminated in the overcurrent device there is this slight possibility you could have a short circuit and an arc flash. So we need to put everything connected to that 600A feeder at risk of shutting down for any fault because of that possibility.

I guess that’s why they have the selection “Including Main”, as I’m not the first one to question this logic.

I just put in a new panelboard at my house. The Incoming wires terminate right in the main breaker and there are plastic covers that go over the inset bolts. Seems it would be very difficult to have a fault before the main. It’s the same situation.

 

[dpc]

Arc flash propagation is real. You have to decide if you're going to calculate incident energy on the load side or the line side of the main device. Always a judgement call. I wouldn't assume that is why they are using instantaneous trip on all devices. It's quite common - often done purely out of ignorance of the coordination tradeoffs. In systems that only have molded case breakers, there's no choice.

 

[cranky108]

If it were a higher voltage, we would use a differential. But maybe we don't want to pay for it.