Emergency stop button behind fuse

Julio Chadrone

Julio Chadrone

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Location
OH
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Controls Engineer/Electrical Engineer
I am designing a panel that I need to be UL 508A complaint. This panel needs an emergency stop button. Which I have tied in line with shunt. The shunt has an SCCR but the pushbutton does not. Since the circuit it's on (just shunt and pushbutton) doesn't draw current I want to call it a control circuit, but UL 508A specifically states control circuits must be behind overcurrent protective devices to be labeled as control circuits. But I don't know how I feel about have an emergency stop behind a fuse because If for some reason the fuse blows that emergency stop button will be in-operable. I also can't imagine I am being the first and only person seeing the issue of having a decently SCCR rated control panel with an emergency stop button, so I'm thinking there's something I'm missing
 
I am emphatically not a professional in the field of safety stop systems.

But I think the way to deal with this situation is to use a NC pushbutton with an undervoltage release rather than a NO pushbuttion with a shunt trip. If the fuse blows then the pushbutton circuit loses power and the release opens the breaker.
 
winnie said:
I am emphatically not a professional in the field of safety stop systems.

But I think the way to deal with this situation is to use a NC pushbutton with an undervoltage release rather than a NO pushbuttion with a shunt trip. If the fuse blows then the pushbutton circuit loses power and the release opens the breaker.
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Under voltage release could cause the need for a reset even though an Emergency stop was not needed, maybe due to something as simple as a POCO power blip.

The use of the phrase Emergency Stop is not always descriptive. Many cases require a simple Power Off circuit rather than a true Emergency Stop via a Listed monitored Safety System.
 
retirede said:
I believe that if it’s an Emergency Stop, it has to be fail-safe (normally closed).
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that sounds about right, I'm still stuck on how to get it to work, I would imagine there is some sort of time delay power relay, that way the shunt doesn't fire immediately on start up
 
jim dungar said:
Under voltage release could cause the need for a reset even though an Emergency stop was not needed, maybe due to something as simple as a POCO power blip.

The use of the phrase Emergency Stop is not always descriptive. Many cases require a simple Power Off circuit rather than a true Emergency Stop via a Listed monitored Safety System.
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Yes, something that will shut off the circuit immediately, and reliably when the button is pressed.
 
Julio Chadrone said:
Yes, something that will shut off the circuit immediately, and reliably when the button is pressed.
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But does it need to be a true Safety system, like described in NFPA79, or is it just someone's generic phrase?

I would probably go with the UV trip with a power loss ride through supply.
 
jim dungar said:
But does it need to be a true Safety system, like described in NFPA79, or is it just someone's generic phrase?
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Oh yeah just someone's generic phrase. That can shut down an mccb, but I will look into a uv trip ty
 
Julio Chadrone said:
that sounds about right, I'm still stuck on how to get it to work, I would imagine there is some sort of time delay power relay, that way the shunt doesn't fire immediately on start up
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How about an N/C push-button holding an N/C relay energized open, that would trip the shunt?

An N/O contact on the same relay in series with the control circuit should provide its own delay.

(I'm really guessing here without more information.)
 
Julio Chadrone said:
Oh yeah just someone's generic phrase. That can shut down an mccb, but I will look into a uv trip ty
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I try to call this a Remote Power Off.

Our industry throws around the word "emergency" too easily, I feel it should be saved for special occasions like Article 517, Article 700, and NFPA 79 type control circuits.
 
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jim dungar said:
I try to call this a Remote Power Off.

Our industry throws around the word "emergency" too easily, I feel it should be saved for special occasions like Article 517, Article 700, and NFPA 79 type control circuits.
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Okay I responded too quickly on this. Yeah, this emergency stop does have to accommodate NFPA 79, I just hadn't fully read NFPA 79 until now since I was going by UL standards, but they essentially say the same thing. Except UL needs that extra thing for SCCR that is messing me up


LarryFine said:
How about an N/C push-button holding an N/C relay energized open, that would trip the shunt?

An N/O contact on the same relay in series with the control circuit should provide its own delay.
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Okay so N/C relay with a N/O relay in series to create the delay I need, will help since finding time delay relays was a challenge. I did end up find some power relays that had an SCCR that was acceptable but they needed to be protected upstream by a fuse so I might just need to throw a fuse between the mccb and distribution block to get UL rating. That way even if that fuse blows the whole system will still be shut down
 
I’m not sure how SCCR is coming into this…

SCCR does NOT apply to control circuits, unless the controls see being done at LINE VOLTAGE. If you have a control power transformer or power supply providing your control circuit, nothing after that falls under the SCCR requirements.

Yes, a control power transformer must have primary protection because that does connect to the LINE voltage side, that is always the case. This is why it’s generally considered a poor design to rely on a shunt trip in a circuit breaker as an Emergency Stop from a machine safety standpoint. A true E-Stop must remain able to act regardless of other things failing and/be “fail safe” in that if something else fails (like a fuse blowing), the machine stops. A shunt trip must TAKE action to stop the machine and there are many things that can interfere with that.

That said, shunt trips can be can be and are utilized for some situations, so long as a risk evaluation has been performed to make sure all parties understand the possibilities of it not functioning if there is a blown fuse or power loss. Ansul systems, elevator controls, and “Remote Power Off” (RPO) systems, which is different from an “Emergency” stop, are a few cases in point.

Under voltage trips are not a good solution, because they trip EVERY TIME power is turned off, even normally, and must be reset, which can cause other problems.
 
Jraef said:
hat said, shunt trips can be can be and are utilized for some situations, so long as a risk evaluation has been performed to make sure all parties understand the possibilities of it not functioning if there is a blown fuse or power loss. Ansul systems, elevator controls, and “Remote Power Off” (RPO) systems, which is different from an “Emergency” stop, are a few cases in point.
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That makes a whole lot of sense. I think I might stop using Shunts as emergency stops then. With everything everyone said, how it should fail on etc, I'll go see what other options are out there
 
I just dealt with something similar but different. My client & friend is building a solarium with a 2000 gal. spa. There's a sliding powered floor that covers it when not in use. So there's the 'equalpotential" area to deal with. The area is fed with a dedicated 200A SqD panelboard.

I insisted on a EPO function & got a shunt-trip main breaker but they do not come with 24VAC coils. The shunt trip coil has its own internal NC switch so it can be live for mere millseconds. (In fact, SqD says it's 120/208/240V coil which baffled me at first.)

The allegedly waterproof EPO pushbutton is outside the 5 ft but I was still dissatified.

So I have a 40VA 24V transformer. The primary is in series with the shunt trip coil. The EPO is across the 24V secondary; when it shorts the secondary, the primary saturates, the shunt trip fires, and the 200A opens.

The shunt trip is fed from a dedicated non-GFI breaker in the panel but if it trips, the EPO isn't. So I have a pilot LED across the EPO contacts, and a sign shall say "No LED, no swimming" or such.
 
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