Control Enclosure Interlock

Status
Not open for further replies.

Elifyno

Member
What does NFPA 79 6.2.3.2 mean? Is interlock being used interchangeably with disconnect? Do you have to build control panels with an enclosure interlock or not? Would it be possible to have an interlock on an enclosure but the disconnect is remote? Some companies are promoting hp rated plug/receptacles as disconnects how does this provide a control enclosure interlock?
 

dlhoule

Senior Member
Location
Michigan
Re: Control Enclosure Interlock

What does NFPA 79 6.2.3.2 mean? Is interlock being used interchangeably with disconnect? Do you have to build control panels with an enclosure interlock or not? Would it be possible to have an interlock on an enclosure but the disconnect is remote? Some companies are promoting hp rated plug/receptacles as disconnects how does this provide a control enclosure interlock?
No they are not. Yes a control panel needs an interlock. Yes you could have a remote disconnect. Trying to keep things simple: if a control panel has a disconnect on it, it has to have an interlock on it to prevent access with the disconnect turned on, unless a tool is used by a qualified person (normally a licensed electrician) to gain access. I believe the intent is to make all equipment safer by only allowing qualified people to gain access to anything over 50V.

The plugs you are referring to IMHO are only to be used to supply power to entire machine or to povide individual disconnect for a motor or some other specific device. With the plug revoved there is no access to live parts. Properly used they provide means for LOTO. and for some applications are very cost effective. :)
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: Control Enclosure Interlock

NFPA79 6.2.3.2 does not require any interlock between a control panel and a remotely mounted disconnect.

What is required is:
A non quick release latching mechanism. Actually it says a key or tool must be used. An example is a standard NEMA 12 enclosure.
or
An if a quick release latch is used then an internal dead front must be provides. An example is a standard 24hr time clock.

NFPA79 6.2.3.1 says only an internal disconnect must be interlocked with the control pnael doors.
 

Elifyno

Member
Re: Control Enclosure Interlock

Thanks for responding. NFPA 79 5.3.1.4 and 6.2.3.1 both say interlock is required if "within or adjacent". The definition of adjacent is not defined but is that to say a disconnect is mounted next to your cabinet it has to interlock with the cabinet? How far away before its considered remote?
 

dlhoule

Senior Member
Location
Michigan
Re: Control Enclosure Interlock

Jim, Good answer. Good explanation.
NFPA79 6.2.3.1 says only an internal disconnect must be interlocked with the control panel doors.
:)
He asks a good question about what constitutes adjacent. IMO If, disconnect is mounted on, as opposed to within, it would be considered adjacent. Beyond being mounted on IMO it would be considered remote.

I still feel intent is to protect everybody from coming in contact with live parts. I would hope anybody working near such a panel would have enough training to know to call for a qualified person to do any work inside control panel.
 

Elifyno

Member
Re: Control Enclosure Interlock

We have machines with the disconnect next to the panel with no interlock.
5.3.1.3 says the disconnect has to be "within the control enclosure or immediately adjacent" unless its a plug or 2 hp or less.
5.3.1.4 says if the disconnect is mounted "within or adjacent" it "shall be interlocked" "in accordance with 6.2.3".

I'm drawing the conclusion that machines over 2hp that do not have a properly rated plug style disconnect must have an interlocked enclosure. Is that correct?

I can see that this could be an arc flash issue. Would it be correct to say that energizing an open panel with a disconnect that is next to it (but not interlocked) exposes you to risk that a remote plug disconnect would not? Was that the intent of the interlock requirement?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: Control Enclosure Interlock

NFPA79 5.3.1.4 goes on to say that if the disconnect is not adjacent then interlocking is not required if a tool is needed to open the door and a warning label is attached.

This is a "contact with live conductors" issue.

I believe 5.3.1.3 requires all machines to have an "interlocked" main supply disconnect unless allowed by 5.3.5. And that 5.3.1.4 and 6.2.3.2 address the circuits allowed in 5.3.5 as well as control cabinets fed by the "main" enclosure.
 

dlhoule

Senior Member
Location
Michigan
Re: Control Enclosure Interlock

This is a "contact with live conductors" issue.
This is also an arc flash issue. If you are standing in front of an open panel with the disconnect on the side and turn on disconnect with a fault you can have a serious arc fault.

This is one reason I think the door on panel should be required to be latched before turning disconnect to the on position. Currently, it is only required to be closed. Though, at one time it was also required to be latched. :)
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: Control Enclosure Interlock

Door interlocking is meant to restrict contact with live parts. Being near live parts means there is a possibility of electrical shock and arc flash.

The NFPA79 sections we have been discussing deal with restricting access to live parts except when you have the proper key or tool. Other sections in NFPA79 deal with warning labels just like some sections in NFPA70. Arc flash is an "Electrical Safe Work Practice" issue and not a manufacturing or construction issue that is one reason there is a seperate standard, NFPA70E.

Also my reading of NFPA79 5.3 says that when the enclosure door is open a disconnect can not be "turned on" without defeating an interlock. Most flange mounted mechanisms have this interlock but I have never seen a "thru-the-door" operator with one.
 

dlhoule

Senior Member
Location
Michigan
Re: Control Enclosure Interlock

Jim, we are in agreement. However, I feel that the door should have to be latched as well as closed before operating disconnect.

One more thing! Please have a great Christmas and a safe and prosperous New Year! :) :) :)
 
Status
Not open for further replies.
Top