Control Enclosure Interlock

[drumburg]

We build machines using supply voltages of 120Vac, 220Vac, and 480Vac. On our machines with 220+Vac we use an NFPA79 compliant Eaton disconnect. These machines are big enough to not have to worry too much about the added cost and space requirements.

However, our smaller 120Vac powered machines, usually tabletop, don't have room for a large disconnect to interlock the doors. I've reviewed the NFPA 79 2018 edition and just need a helping hand to see if we can use alternatives to an interlocking disconnect for these smaller machines.

According to 5.3.1.4 we would be required to interlock the control enclosure door but Exception No. 1 looks like we can use a key/tool removable dead-front cover to eliminate the interlock. My question here is: Are there specific requirements for a cover to be considered a dead-front cover other than preventing contact with exposed live parts? Such as material or construction. Does the enclosure also have to have a door or can we just use a bolt-on cover as the dead-front cover?

For another alternative, I have read in other forums that if all components in the control enclosure are IP20 (i.e. Touch Safe) then an interlock is not required although I haven't found that specifically in the NFP79 document.

 

[petersonra]

I would think with a 120 volt powered system you would not need a "large" disconnect. A 30 Amp IEC style disconnect switch does not take much more space than a pushbutton.

 

[drumburg]

The last time I had talked with Eaton about what disconnect to use for a 120V system they recommended the same one we use for our 220Vac. That would take up almost 1/3 of the enclosure space. Maybe there is a smaller one.

I have a reasonably sized power switch to use with the 110V systems we build but it doesn't interlock the door. Do I need to interlock the door if I can use a dead-front cover or use IP20 devices?

 

[paulengr]

What purpose does the interlock serve? Why the concern about NFPA 79 which is wholly voluntary? Sounds like Eaton simply has no product available.

It costs very little to install a circuit breaker or fuserbloc. Then mount an adapter frame and rod on it. Cut a hole in the door and mount the rotary handle disconnect. The big trick is using a cross line laser to perfectly line up the hole in the cover and cutting the rod exactly to length. These things are tiny at lower voltages and the disconnect handle captures the rod and will not allow the door to open in the on position. There are tons of these on the market. Look at say Benshaw for instance.

 

[petersonra]

Do you always let vender sales people do your design work for you?

here is an AB smallish disconnect switch for 30 A class J fuses.


It is about 5" across and fits on the back plate of the enclosure. A shaft goes from the switch, through the door, and into a handle. The handle is on the outside. It has an interlock on it that grabs the door so it cannot be opened without a tool.

These things are not real expensive. Under $200 including switch, shaft, handle, and fuses. Other companies have similar products that cost even less.

 

[Jraef]

How many amps are we talking about here? Is the 120V for powering something outside of the enclosure like a motor or heater, or is it just controls? If it's just controls, I'm not sure you need door interlocking. If there needs to be a door interlocking handle, I know Siemens makes one that fits onto their little MCBs (Miniature Circuit Breakers). It's made to accommodate up to a 3 pole breaker but it works for a 2 pole too, I've done it (I know you only need 1 pole, but this is what they have). The exterior handle is still the same as all of the other 30A disconnects, but on the panel it's smaller.

ABB makes a version too, they just don't show the handle, but again, it's the same handle and shaft they use on their small rotary disconnects.

 

[drumburg]

As far as us complying with NFPA 79. I've read on this forum that NFPA 79 is voluntary and maybe only enforced by OSHA for end users of purchased equipment. However, my department builds equipment for use within the our own company so if OSHA forces the end user to follow it then that would probably include us as well. We are not a UL panel shop so we use NFPA 79 as our standards to follow. Regardless of whether or not we must, we have decided to comply with NFPA 79 and it does provide us a place to start at least.

By following NFPA 79, I believe we would need to interlock the control enclosure door because of 5.3.1.1 "A supply circuit disconnecting means shall be provided for the following: (1) Each supply circuit connecting to a machine" and 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 6.2.3.1." 6.2.3.1 says to interlock the enclosure if the voltage is above 50Vac or 60Vdc. Our voltage is at 120Vac.

The control enclosure will have a VFD to run a conveyor motor, a 24Vdc power supply, a PLC, and other basic devices.

I asked Eaton for what they would recommend because I figured they would know better than I did without having to spend too much time digging through their product lists to find something. I guess at the time we were using Eaton and I just didn't think to look elsewhere, my mistake.

Thank you for these items that fit/mount directly to circuit breakers. I did not know that these products existed. I will definitely do some checking into them and see how close they are to NFPA 79 compliant.

 

[Jraef]

So to be NFPA compliant, they must also have a means to operate the disconnect when the door is open, without tools. So most of the mfrs have an "NFPA compliant handle" that you can add to the shaft on the inside of the enclosure.

 

[drumburg]

In the interest of keeping people safe without pricing ourselves out of the market, what is an alternative to interlocking the enclosure?

If NFPA 79 is not mandatory what standards do other shops use to build their machines to?

Hypothetical system:
12x14x8 Single Door, Tool Entry Enclosure housing a 24Vdc power supply (120Vac input), circuit breaker, power switch, and a PLC controlling some solenoid valves (mounted externally to the enclosure). Do I need an additional enclosure interlock to keep people out of the enclosure? If not, is there some standard that I can reference?