Shunt trip w/o a shunt trip breaker

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massfd

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I thought I had seen it all relating to cooking hoods and supression system shutdowns but this one is new to me. AHJ requires power to be shutdown to all electric cooking equipment on system discharge. This is normaly done with a shunt trip breaker.

Today I saw it done with a GFCI breaker, they brought the neutral and ECG to the cylinder control head and put the wires on the microswitch. On system trip the microswitch connects the neutral and ECG and the GFCI trips.

Now I know (or at least I think I know) that the GFCI is not listed to work in this manner but I still think it is a great idea for 1 reason. Normaly, with a shunt trip breaker you have the shunt power supply voltage in the microswitch head. Along with this voltage you may also have low voltage from the building fire alarm system on the next switch less than 1/4" away. Using the GFCI the only thing you have on the microswitch is a neutral and a ground, how can you get hurt or blow up a firealarm system with that?

What do you think, if a manfacturer got a listing for this type of function would you use it to eliminate line voltages in the microswitch cabinet?
I know you could power a shunt trip breaker and switch the neutral to get almost the same thing but I do not think code allows switching the neutral and besides that when the microswitch is open you still have a line voltage (in series with the shunt coil) in the cabinet.
 
massfd said:
Code references based on 2005 NEC

Larry

I love your answer combined with your tag line it just looks odd
Click to expand...
Hey, if the shoe fits . . . :D


I prefer RIBs or contactors for controlling cooking equipment. Less expensive than adding ST's, especially in existing kitchens, and especially if there aren't already some in use.
 
hardworkingstiff said:
What's a RIB Larry?
Click to expand...


RIB = Relay in a box

hvac_button_test.gif



P-RelaysSensors.jpg


http://hvac.functionaldevices.com/chartRelays.html


It's like an Ice Cube relay but you can put them just about anywhere.

EDIT:
Here is a typical ice cube:
relay24.gif
 
Last edited:
LarryFine said:
What he said:
The best thing about the RIB is it's easy to mount, especially to interrupt a circuit in an existing J-box or panel KO, and easily terminated w/wirenuts.

Gee, I guess that's just about everything about the RIB. ;)
Click to expand...


RIB has 600v insulation
icon14.gif
 
massfd said:
Today I saw it done with a GFCI breaker, they brought the neutral and ECG to the cylinder control head and put the wires on the microswitch. On system trip the microswitch connects the neutral and ECG and the GFCI trips.

Now I know (or at least I think I know) that the GFCI is not listed to work in this manner but I still think it is a great idea for 1 reason. Normaly, with a shunt trip breaker you have the shunt power supply voltage in the microswitch head.
Click to expand...


I would question whether the microswitch was rated for the low current value of the GFCI.
What is the normal current for a shunt trip? Now compare this to the 6 ma required to trip the GFCI. Unless the microswitch contacts have a gold flash they may not operate reliably with this low of a current.

Although innovative I cannot see this passing safety standards.
 
ELA said:
I would question whether the microswitch was rated for the low current value of the GFCI.
What is the normal current for a shunt trip? Now compare this to the 6 ma required to trip the GFCI. Unless the microswitch contacts have a gold flash they may not operate reliably with this low of a current.

Although innovative I cannot see this passing safety standards.
Click to expand...

Shunts don't operate via current...it's a coil that responds to a voltage.
 
ELA said:
I would question whether the microswitch was rated for the low current value of the GFCI.
What is the normal current for a shunt trip? Now compare this to the 6 ma required to trip the GFCI. Unless the microswitch contacts have a gold flash they may not operate reliably with this low of a current.
Click to expand...

FYI, a 120VAC shunt trip coil for a Square D QO breaker pulls 72VA or about 600mA.

But, in this case, the current flowing through the neutral to ground connection would be based on the resistance of the circuit and the voltage between N-G. The GFCI does not draw 6mA, rather it responds once this amount of current is flowing through the neutral and not through the hot conductor.
 
celtic said:
Shunts don't operate via current...it's a coil that responds to a voltage.
Click to expand...

Coils pick on a voltage but will not operate if current is limited too much.

In this discussion we are concerned with current they require as Jim was kind enough to provide.
 
jim dungar said:
FYI, a 120VAC shunt trip coil for a Square D QO breaker pulls 72VA or about 600mA.

But, in this case, the current flowing through the neutral to ground connection would be based on the resistance of the circuit and the voltage between N-G. The GFCI does not draw 6mA, rather it responds once this amount of current is flowing through the neutral and not through the hot conductor.
Click to expand...

We are not talking about the current demand of the GFCI. If I understood correctly they are shorting the neutral to the gnd and taking advantage of the circuitry within the newer GFCIs that detect the downstream neutral to gnd fault.
This circuitry utilizes the same sense coil within the GFCI and senses ~ 6 ma.

Thanks for supplying the typical 600ma value.
My point was not enough current to clean the contacts.
 
whaaatttt

whaaatttt

whaattt

Amp clamp the panel neutral current and guesstimate that half of this current will flow on a parallel path if created. Shorting neutral to EGC creates this parallel path and some neutral current will flow on the EGC then to the EGC to X0 bonding jumper on its way back to the source transformer secondary neutral.

1. Available neutral current could easily exceed the microswitch rating, especially if routinely applied in diverse installs.

2. NEC violation creating a parallel path for system neutral current on the grounding paths. The microswitch is not momentary contact. It makes until reset.

3. UL violation using a UL listed device outside of its UL tested arrangement.

4. If you still like this, install a 500 ohm, 1 watt resistor in series with the neutral to EGC path to limit neutral current to a acceptable value large enough to pass the detection threshold of the GFI.
 
ELA said:
I would question whether the microswitch was rated for the low current value of the GFCI.
What is the normal current for a shunt trip? Now compare this to the 6 ma required to trip the GFCI. Unless the microswitch contacts have a gold flash they may not operate reliably with this low of a current.

Although innovative I cannot see this passing safety standards.
Click to expand...

Turns out my memory is short :confused: and I did not need to question the current level required for the GFCI (Neutral-PE) downstream short sense circuitry to activate. I had tested this a while back. Here is the scope trace from a zero ohm short from Neutral to PE, on the load side -with no load devices or current external to the test short.

http://i268.photobucket.com/albums/jj22/transorb/LoadNeu-PE0ohms.jpg


Although the sense coil senses anything greater than 6 ma ...
This shows that the injection coil circuit used for this feature of the GFCI overdrives the sense coil to assure a minimum of 6 ma even when the downstream resistance is as great as 30 ohms.

As shown - with a zero ohm short - the current is nearly in the same range as
with a shunt trip coil.
So no problem as far as a minimum load on the contacts.
 
Oops Please ignore my rantings.
I got hung up on the concept of sensing a GFCI Neutral to PE short- without a load - and failed to envision the entire circuit.
After thinking on it I get it now. In this case I agree that if anything the concern may be too much current through the contacts depending upon what the load is.
 
The GFCI breaker is only good for one circuit, many hoods have more than one circuit that

needs to be opened, but, if only one circuit is involved, just run the circuit thru the Ansul

contacts and never mind inventing things when peoples' lives are involved.

It may be clever but it is as Larry said " nuts".
 
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