UL508a use of electrical interlocks

[petersonra]

I have been having a discussion with several fellow engineers here about the use of electrical interlocks versus mechanical interlocks for UL508a ICP disconnecting means. This came up because a project may end up needing a 2500A main CB and they do not have the common mechanical means by which to interlock them with the enclosure doors, so the use of a spring loaded solenoid lock with a key operated switch to release the solenoid.

Here is the relevant UL508a section.

66.1.5 A door of an enclosure that gives access to uninsulated live parts operating at 50 volts rms ac or
60V dc or more shall be interlocked with the disconnecting means such that none of the doors can be
opened unless the power is disconnected.
Exception: A disconnecting means for maintenance lighting circuits or for power supply circuits to control
devices with memory requiring power at all times are not required to be interlocked with the enclosure
doors. The cautionary marking in 55.4 shall be provided.
Revised 66.1.5 effective March 1, 2007
66.1.5.1 The interlocking means required by 66.1.5 shall be provided with all the following:
a) Means to defeat the interlock without removing power and which requires the use of a tool to
operate;
b) Means to prevent restoring power while the enclosure doors are open unless a defeat
mechanism is operated; and
c) Reactivated automatically when all the doors are closed.

It appears to me that a key operated solenoid locking mechanism (spring loaded to unlock) clearly fails 66.1.5.1 on both clause a and b. This is my logic.

Read the requirements carefully, and then consider what would happen if the control circuit to the lock were to fail (say the fuse blows).

The main CB would still be closed but the interlock would be defeated, thus violating the requirement in clause (a) that a tool be required to defeat the interlock.

The same problem exists with clause (b). If the control circuit is de-energized (say that someone was working on the circuit and forgot to turn it back on), the interlock will not re-engage when the door closes.

 

[Smart $]

... so the use of a spring loaded solenoid lock with a key operated switch to release the solenoid.

...

I agree. Does not meet requirement.

 

[Jraef]

Just out of curiosity, who's 2500A CB are you using that has no door interlocking operating mechanism?

 

[petersonra]

Just out of curiosity, who's 2500A CB are you using that has no door interlocking operating mechanism?

As best I can tell from the manufacturers, none of them come with a way to mechanically interlock it with a std industrial style enclosure with an externally accessible handle.

I learned something new. Above 1600A, the CB makers do not make MCCB. Siemens used to make one at 2000A but stopped.

Above 1600A they are called "air breakers", a term I had not heard of before. They no longer have an operating handle that a remote operator can use to operate the CB. They have a couple of buttons - one to trip and one to reset. The two I looked at also had a handle that one must use to pump up the spring (or something) before you can reset it unless you buy it with a motor operator built in.

I am now considering whether putting a single CB inside a cabinet consitutes an industrial control panel or not. Since it is just one device, I think it is not an ICP.

I am also considering if there is some way to bring the feed into a seperate enclsoure and split it into two 1200A feeds that I can get a common MCCB for, and split the ICP into two ICPs. To me the seperate enclosure is really a gutter and not an ICP.

It is a non-trivial exercise to find some way to distribute power at those levels.

 

[Jraef]

Huh, not sure what happened to my post from Saturday, must have been lost in transmission. Postus Interruptus I guess...

The Eaton Series G RG frame MCCB goes up to 2500A and has a cable operated flange-mount disconnect handle option. It only has rear connection capabilities so you need an enclosure with rear access to be able to make connections. In a job once where I had no rear access once it was installed, I used Flexi-bus going to some Ilsco stud connectors that allowed for use of large enough mechanical lugs for the field cables, that way the rear connection capability was only required in my shop. If someone ever had to replace that breaker they were likely cursing me though...

 

[jim dungar]

They no longer have an operating handle...

Don't know where you are getting this from. Jreaf mentioned one, here is another. Schneider Electric's Square D Powerpact R-frame is handle operated up to 2500A, although I do not believe they offer a remote operating handle for it.

Power 'air', as opposed to vacuum or gas filled, circuit breakers operate using a charging mechanism and a set of springs to open and close. The springs are 'pushbutton' actuated. These have been around for decades, at one time they were called 'iron-frame' breakers.

 

[petersonra]

Huh, not sure what happened to my post from Saturday, must have been lost in transmission. Postus Interruptus I guess...

The Eaton Series G RG frame MCCB goes up to 2500A and has a cable operated flange-mount disconnect handle option. It only has rear connection capabilities so you need an enclosure with rear access to be able to make connections. In a job once where I had no rear access once it was installed, I used Flexi-bus going to some Ilsco stud connectors that allowed for use of large enough mechanical lugs for the field cables, that way the rear connection capability was only required in my shop. If someone ever had to replace that breaker they were likely cursing me though...

Flexibus would be nice but it is not in our UL procedure just yet, so we cannot use it in a 508a listed panel. It is on my list to get it added.

 

[Jraef]

Flexibus would be nice but it is not in our UL procedure just yet, so we cannot use it in a 508a listed panel. It is on my list to get it added.

You could do it with multiple cables and crimp lugs too, Flexibus is just easier to work with, especially for making tight bends.

 

[petersonra]

You could do it with multiple cables and crimp lugs too, Flexibus is just easier to work with, especially for making tight bends.

That is mostly what we do now. It is not too awful for smaller stuff, but you get to where you are lugging on 4 or 5 beig cables and it is a mess. Flexibus is the way to go IMO, but it has to get added to our procedure first.