Medium Voltage Switchgear Breakers

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Many do, but we don't know what you are looking at. Some have a programmable interface, usually referred to as an "ETU" for Electronic Trip Unit. Some older ones have a trip unit that requires a "rating plug" for the value that you want, so it is "adjustable" by virtue of what rating plug you install. Older yet were ones that had electro-mechanical trip relays on them and yet others have NO trip unit attached to the breaker itself, they are meant to interface with "Feeder Protection Relays" that then shunt-trip the breaker.
 
AND adding to that most medium voltage equipment is made to order, nonstocked. So it is highly customizable. Certain breakers for instance are standardized so they are interchangeable and others are customized to the point that the breaker wiring is customer specified like GE. Trip units can go either way. too.
 
I think I figured it out- 1200 Amp would be the high limit of continuous rating the breaker is designed to carry. Interfaced with protective relays, these MV breakers can protect feeders carrying lower Amps by virtue of adjusting long time delay trip setting of the relay curve.
 
FaradayFF said:
I think I figured it out- 1200 Amp would be the high limit of continuous rating the breaker is designed to carry. Interfaced with protective relays, these MV breakers can protect feeders carrying lower Amps by virtue of adjusting long time delay trip setting of the relay curve.
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That is the case. I don't think I could tell you that I've ever seen a MV breaker designed with an integral trip unit.

It is always a relay (whether an older dedicated protection type 50 (instantaneous only), 51 (long time only), etc OR multifunction such as an SEL 751 or Multilin 750) that trips the shunt on the MV breaker with a (87) lockout. The relay is adjustable with selected CTs to any value less than the 1200A rating of the breaker.
 
ron said:
That is the case. I don't think I could tell you that I've ever seen a MV breaker designed with an integral trip unit.

It is always a relay (whether an older dedicated protection type 50 (instantaneous only), 51 (long time only), etc OR multifunction such as an SEL 751 or Multilin 750) that trips the shunt on the MV breaker with a (87) lockout. The relay is adjustable with selected CTs to any value less than the 1200A rating of the breaker.
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GE RMS-9 uo to Intelligard is on some of them.

The newer ones don’t use an 87 as it adds 1 cycle delay. The 87 is part of the 750/751/751A software.
 
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