NFPA 1

[patm72]

A cable (A) feeding an electric 3-ph motor using a softstart can be disconnected from a machine. Another cable (B) feeds control voltage to the same part of the machine. When this latter cable (B) is connected, it enables a control contactor feeding the 3 phase voltage to the softstart and the motor (it's not a start signal).

Upstream, I have overcurrent protection (3 fuses, 30A) for the motor.

So in panel: 30A Fuses - Contactor - Softstart
From panel to equipment: Cable (A) - Connector - Motor
Control Cable (B) - Connector - Junction box

A colleague wants to insert a breaker equipped with a special electronic trip unit between the fuses and contactor which is to trip when control cable (B) is disconnected. It would then be rearmed by push-button when the cable is connected.

I feel I am already protected by the contactor being disabled when (B) is disconnected, and disagree with him. NFPA79 art. 5.4.2 backs up my reasoning and to a lesser extent, cost.

He is concerned about voltage possibly being applied to the connector at the end of the disconnected cable (A). The connector is already fingersafe.

Any thoughts on this?

 

[jraef]

Is there anything preventing you from disconnecting (A) even if (B) is still connected? If not, he has a valid concern because if (B) is left connected, the contactor is closed and there is potential on the ends of cable (A), regardless of your intent. And by the way, the term "finger safe" is not used in NFPA79, so that doesn't count for anything officially. The connector pins would have to be considered dead, also called "dead front" (for lack of a better term).

I'm not sure how you would do this with a "electronic circuit breaker" any differently than it is already being done now however. If you use a Under Voltage trip, it is not going to know that the plug has been disconnected; it will only be able to connect to and monitor the line side. If it is a shunt trip, you STILL need a command to trip the breaker, and the same command could also be used to open the contactor as you have it now. I can't think of any other "electronic circuit" that could monitor whether or not the load is connected.

There are 3 ways of approaching it that I can think of;
1) Put a "pilot circuit" through another set of pins (assuming you have a pin and sleeve connector?) on (A) so that as you begin to disconnect the cable, before the main circuit pins disconnect, the pilot circuit does and drops out the contactor.

2) Mechanically interlock (A) and (B) so that you cannot disconnect (A) unless (B) is already disconnected. There are connectors like that. Sometimes it is done with a mechanical shutter type mechanism, you can also do it with Kirk Keys.

3) Use pin and sleeve connectors that are rated as disconnect devices. Meltric is one mfr I know of, maybe the only one. Their caps and connectors are spring loaded, just like a contactor, and rated to be pulled apart under load. That way even if someone forgot to disconnect (B) first, the end of the cap is considered "dead front" when disconnected.

http://www.meltric.com/?gclid=CPH05sCi4pkCFRwpawodrViDVQ

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