Emergency stop polarity?

[Tom Shaughnessy]

I am looking for information on switching in a 24V emergency stop circuit. is there a code requirement for switching the 0V side or 24 volt side? I am reviewing a piece of equipment and find the estop button is wired on the 0 Volt side of the circuit. In this case the 0 volt or neutral side of the circuit is grounded. in this case a short circuit would be like jumping the circuit out, where if controlled on the +24volt side the shorted condition would cause a breaker to trip and shut down. Before pushing for a change, i wanted to be certain it is not a requirement for some other reason. Any recommendations?

 

[hbiss]

Nothing in the Code specifies, but common thinking would be to switch the ungrounded (hot) side.

-Hal

 

[petersonra]

What is this E stop controlling?

 

[zbang]

Is this an already-listed piece or something custom?

 

[retirede]

Failsafe design would always dictate having the E-stop in the ungrounded conductor of the circuit for the reasons you mention.

 

[petersonra]

Failsafe design would always dictate having the E-stop in the ungrounded conductor of the circuit for the reasons you mention.

I'm inclined to agree with you for the most part however I don't think it makes a whole lot of difference if it's a shunt trip type breaker that's being tripped by the estop pb.

If loss of power to the main coil is what trips the circuit probably a good idea. I guess it kind of depends on what kind of emergency stop it is. I wouldn't get too excited about some things but some things I might.

 

[don_resqcapt19]

If this is a motor control circuit, see 430.74. Even if it is not a motor control circuit, it is not a good idea to wire the circuit in a manner that would permit a ground fault to disable the E-Stop.

 

[texie]

The OP indicates this is 24 volt. I'm wondering if this is a class 2 circuit and if it is even grounded to make this question relevant.

 

[retirede]

The OP indicates this is 24 volt. I'm wondering if this is a class 2 circuit and if it is even grounded to make this question relevant.

The way his question is worded would seem to indicate that it is grounded.

 

[jim dungar]

Could it be that this is a redundant contact style E-stop, with contacts in both hot and grounded conductors. I think this is common with safety relays.

 

[ptonsparky]

We would wire the Stop so it would break both conductors, totally isolating the control circuit.

 

[LarryFine]

This reminds me of the discussion about O/Ls wired in the un-switched side of motor-starter coils.

 

[petersonra]

Could it be that this is a redundant contact style E-stop, with contacts in both hot and grounded conductors. I think this is common with safety relays.

Actual safety relays don't wire this way at all.