NFPA 79 Industrial Enclosure Interlocks

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drumburg

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Hi Gang.

I am working on trying to find out when to use enclosure interlocks and what is a decent one that I can get the most bang for my buck. I have reviewed the NFPA 79 chapters 5 & 6 and dug around here a bit on the topic and it seems that the key to whether or not an interlock is required or not is what the enclosure is used for. We build automated equipment and we always have an interlock on our systems main enclosure from the through door rotary disconnect with an NFPA 79 Compliance Kit from the disconnect manufacturer. NFPA 79 states that any control enclosure must have an interlock. What is the definition of a control enclosure? Since our main enclosure is always interlocked via the disconnect, this only comes up when we have remote boxes that house components with >50 Vac or >60 Vdc per NFPA 79 6.2.4.

Specific example, my current project is going to require a remote box hardwired to the main enclosure (no plugs or disconnect points). This remote box will have 230Vac, 3ph to run 4 motor speed controls for the conveyors on the system. The box will also house the safety contactor and the circuit breakers for those conveyors. Does this enclosure require an interlock? Does having a disconnecting plug or switch on this remote box change anything?

Thanks for you time!
 
drumburg said:
Hi Gang.

I am working on trying to find out when to use enclosure interlocks and what is a decent one that I can get the most bang for my buck. I have reviewed the NFPA 79 chapters 5 & 6 and dug around here a bit on the topic and it seems that the key to whether or not an interlock is required or not is what the enclosure is used for. We build automated equipment and we always have an interlock on our systems main enclosure from the through door rotary disconnect with an NFPA 79 Compliance Kit from the disconnect manufacturer. NFPA 79 states that any control enclosure must have an interlock. What is the definition of a control enclosure? Since our main enclosure is always interlocked via the disconnect, this only comes up when we have remote boxes that house components with >50 Vac or >60 Vdc per NFPA 79 6.2.4.

Specific example, my current project is going to require a remote box hardwired to the main enclosure (no plugs or disconnect points). This remote box will have 230Vac, 3ph to run 4 motor speed controls for the conveyors on the system. The box will also house the safety contactor and the circuit breakers for those conveyors. Does this enclosure require an interlock? Does having a disconnecting plug or switch on this remote box change anything?

Thanks for you time!
Click to expand...

I think you are missing something here.

Applies to supply circuit disconnects.

5.3.1.4 Each supply circuit disconnecting means mounted
within or adjacent to the control enclosure shall be interlocked
with the control enclosure in accordance with 6.2.4.
Click to expand...

5.3.1.4 only applies to supply circuit disconnects so 6.2.4 only applies to them as well
6.2.4 Enclosure Interlocking. When required by 5.3.1.4, each
disconnecting means mounted within or adjacent to a control
enclosure that contains live parts operating at 50 volts ac (rms
value) or 60 volts dc or more shall be mechanically or electrically
interlocked, or both, with the control enclosure doors so
that none of the doors open unless the power is disconnected.
Interlocking shall be reactivated automatically when all the
doors are closed.
Click to expand...

So if an enclosure does not include a supply circuit disconnecting means, no interlock is required.
 
petersonra said:
So if an enclosure does not include a supply circuit disconnecting means, no interlock is required.
Click to expand...

That's what I had originally thought as well but we have an electrical guy from another company that works for us now and he was refuting the logic stating that all enclosures on a machine would fall under this clause if the voltages were high enough. I had also talked with another panel shop near us that had stated they never used enclosure disconnects separate from the main disconnect.

So a follow up question, if I were to put a separate disconnect switch on the conveyor controller enclosure, then I would need to interlock the enclosure with that disconnect even if it is part of system and supplied from another enclosure?
 
It’s really the OSHA electrical safety rules and LO/TO requirements you need to worry about, you cannot be allowed to open a panel that has more than 50V to ground without having to disconnect power first. NFPA 79 reiterates that but also adds that the disconnecting means must have an internally accessible operating mechanism for when the door has been opened and a qualified technician has re-energized the disconnect for testing. In the “olden days” it was OK for the technician to use a tool to re-energized the disconnect, but then there were accidents that were the result of power needing to be turned off, but the right tool was unavailable. So NFPA 79 added the internal handle requirement.

What is typically done in situations like yours is that the remote panels will have an electrically operated latch mechanism (ie 120V) that is powered FROM the main enclosure that has the supply disconnecting means. So in order to open the remote panel, the main panel must be de-energized. That will satisfy both NFPA 79 and OSHA requirements including LO/TO because if you have locked and tagged the main disconnect open, all panels fed from it are now safe (assuming no foreign sources of energy). I’ve even had it where we’ve had multiple electric lock units on the same enclosure because of multiple sources of power coming together in the same box.

Having plugs for the loads makes no difference in terms of entering the enclosures, but plugs often can serve in lieu of local disconnecting means for the loads themselves.
 
Jraef said:
It’s really the OSHA electrical safety rules and LO/TO requirements you need to worry about, you cannot be allowed to open a panel that has more than 50V to ground without having to disconnect power first. NFPA 79 reiterates that but also adds that the disconnecting means must have an internally accessible operating mechanism for when the door has been opened and a qualified technician has re-energized the disconnect for testing. In the “olden days” it was OK for the technician to use a tool to re-energized the disconnect, but then there were accidents that were the result of power needing to be turned off, but the right tool was unavailable. So NFPA 79 added the internal handle requirement.

What is typically done in situations like yours is that the remote panels will have an electrically operated latch mechanism (ie 120V) that is powered FROM the main enclosure that has the supply disconnecting means. So in order to open the remote panel, the main panel must be de-energized. That will satisfy both NFPA 79 and OSHA requirements including LO/TO because if you have locked and tagged the main disconnect open, all panels fed from it are now safe (assuming no foreign sources of energy). I’ve even had it where we’ve had multiple electric lock units on the same enclosure because of multiple sources of power coming together in the same box.

Having plugs for the loads makes no difference in terms of entering the enclosures, but plugs often can serve in lieu of local disconnecting means for the loads themselves.
Click to expand...

Can you provide the OSHA code reference that shows this? What you wrote seems very clear but provides no alternative to removing power. I would assume that OSHA knows about troubleshooting equipment and understands that it is sometimes required to enter an enclosure with power on to properly diagnose the fault and would provide provisions for that. What does OSHA allow in these cases?

Interlocking the remote enclosures isn't a huge deal on the surface but finding a suitable interlock has been difficult thus far. I have found one that operates on 120Vac that is pretty cost effective but most of my equipment runs on 230Vac and I haven't found a cost effective alternative. Any suggestions would be appreciated.
 
drumburg said:
Can you provide the OSHA code reference that shows this? What you wrote seems very clear but provides no alternative to removing power. I would assume that OSHA knows about troubleshooting equipment and understands that it is sometimes required to enter an enclosure with power on to properly diagnose the fault and would provide provisions for that. What does OSHA allow in these cases?

Interlocking the remote enclosures isn't a huge deal on the surface but finding a suitable interlock has been difficult thus far. I have found one that operates on 120Vac that is pretty cost effective but most of my equipment runs on 230Vac and I haven't found a cost effective alternative. Any suggestions would be appreciated.
Click to expand...

230V

https://www.pentairprotect.com/en/hoffman/Products/3074457345616789227
 
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