Enclosure locking


Thread Starter

Bruce Axtell

Most multibay enclosures interlock the doors to prevent access unless the main disconnect is off or a tool-operated override is actuated by
qualified service personnel. The disconnect itself is capable of being locked off. (I'm referring to typical NEMA 12 enclosures housing
480-3-60 devices such as starters, drives, etc.)

Is anyone aware of restrictions (OSHA, NEC, NFPA79, etc) AGAINST locking the enclosure doors, irrespective of the disconnect? Or to put
it another way, if padlocks, for example, were installed on the doors to prevent access, would this be a violation? If the disconnect is off,
access would still be denied unless someone was authorized to unlock it. What if (however unlikely) a fire should occur within such an enclosure and the locks prevented access to put it out? I think it is a bad idea, but cannot think of a code that specifically states you cannot lock an enclosure in this manner. Is the type or method of locking defined by code?

My suggestion was to install solenoid locks (similar to those manufactured by Hoffman) that energize when the disconnect is on, and
keyswitch overrides to allow entry by service personnel. If a fire should occur, the first thing one would do is turn off the disconnect
which would then allow access by any person with a fire extinquisher.

I'm not aware of any restriction prohibiting locking an enclosure SHUT. I have seen this done in some large facilities, which are very closely
scrutinized by OSHA and other regulatory bodies, and have seen no violations charged. I am aware of (as you stated) the code which requires the disconnect must be lockable in the OFF position. IMHO, if a fire occurred in an electrical panel, I would want the doors to remain closed!. Once the source of energy is removed (i.e. disconnect) the fire must smother itself, unless air is introduced from another source. In any case, any qualified fire-fighting personnel would surely have the required tools needed to force a small padlock of the size and type I envision you are describing. I have seen the need for locking of
enclosures to prevent accidental and deliberate tampering.
In the past we have put systems into buildings where all doors must remain secure but fire regulations demand that "fail safe" emergency access and egress be provided.

We use Magnalocks (Securitron in the US) which are basically big electromagnets fixed into the doorframe with a plate secured to the door.
They're available in lots of sizes and holding strengths.

They are ideal for our applications as they are reliable, they can't jam (although a coating of SuperGlue renders them completely "fail unsafe" ;), they're difficult to vandalise (no moving parts) and they fail safe (if the power goes off there's no way the door can stay locked).

These would then be controlled either by electronic access control or simply by a normally closed key switch.

For fire regulations we have installed normally closed "break glass" points adjacent to each door with the contacts wired in series with the lock so if there's a fire you smash the glass and the door pops open.

On the outside of the building we would have a "fireman's keyswitch" which is a special key operated switch mounted at high level above the door. All fire appliances in the UK carry these keys which not only open doors like ours but also open lift doors etc.

A point to remember on fire fighter access is to keep things in perspective. If you are providing access for a fire crew you are probably wasting your time. If they want to get through a door, they WILL get through a door, locked or otherwise

There are maybe twenty of these guys with axes and a ten ton truck full of equipment.

If you're providing access for the innocent bystander who might be forced into action, then you'd better give them all the help you can. They've probably never done this before and they're probably panicking.

Geoff Moore
Straight Forward Solutions Ltd
Hi Bruce,

Sorry that I can't help you on your quest to prevent enclosure locking.... I assume that you're trying to stop someone from padlocking

From my take on it, I would guess that there are no laws that prevent the auxiliary locking of enclosures. I came to this conclusion because, when you look through an enclosure catalog, they have "padlockable" latch handles available (look in the "accessories" sections).

Standard interlocking "should" provide sufficient unauthorized access (and personnel safety), but when you're trying to prevent an "electrician wanna-be" from fooling with the equipment, a padlock is sometimes the only

Maybe someone else on the list knows of some "loop-hole" that can help you out....


- Eric Nelson
[email protected]
Packaging Associates Automation Inc. [email protected]
Rockaway, NJ, USA

Paul McGuire

>> but when you're trying to prevent an "electrician
>> wanna-be" from fooling with the equipment, a padlock is sometimes the
>> solution.

How about a "decoy" or "placebo" enclosure, with suitable-looking breakers and other hardware, that might light an indicator light, or throw some
impressive-looking low-voltage sparks, but in fact is not connected to anything? :)

When the serviceman comes, just be sure to point him to the correct enclosure (which has been installed in an appropriately-limited access

Paul McGuire
Sr. Mfg Applications Engineer
ObjectSpace Fab Solutions
Austin, TX
- Advanced Process Control Solutions for the Semiconductor Industry -

James Bouchard

I am a bit behind on my e-mail but I have a few comments on enclosure locking.

The problem with all the large multidoor enclosures is the interlocking mechanism that keeps all the doors mechanically locked until the power is removed and also allows authorized persons to bypass this interlock and open the door with power still on. We have enclosures with 12 3 foot doors and have never been able to keep the interlock mechanism working for any time.
Also on enclosures with a lot of doors, the bypass mechanism does not work very well even when it is working. To open a door any distance from the main switch requires two people, one at the switch to actuate the mechanism and
another at the door in question to activate the interlock bypass. I have seen these problems on enclosures having from 3 to 12 doors and from several manufacturers so it is not just an isolated manufacturing problem.

To avoid all this trouble we padlock all the panel doors and leave the mechanical interlock out except on the door in front of the main switch. The padlocks are all keyed alike for the entire plant ( several hundred of them ) and have a short chain on them so they can be bolted to the panel and never get lost.

We have also specified finger safe connections and additional guards over terminals that are not to minimize the possibility of accidental contact
with live components. We also put viewing windows in front of the PLC racks and other equipment of interest and brought out all the programming plugs to the enclosure door along with a convenience outlet for programming.

We also have a policy of keeping the doors closed and locked unless someone is actually at or at least near the panel. Only the electrical technician and engineers have the key for these padlocks.

This has reduced the number of motor problems caused by repeated resetting of a stalled motor and changes in adjustments and so on. It has also
eliminated the operator using the enclosure as a storage cabinet.

None of the local inspectors have commented adversely on our locking the enclosures, the CSA inspector only required that we put a warning label on the panels about live work, which we did. Our corporate safety inspectors liked our approach very much and have mentioned it to other plants.

James Bouchard