Signaling relays in purged Class I Division 2 enclosure

[justdavemamm]

We have an enclosure in a Class I Division 2 area. It has a type Z purge with monitoring so we do not have automatic power cutoff when the purge is lost, just a local light and a relay from the pressure switch inside that has dry contacts (form 'C', both N.C. and N.O.) that are wired to a remote safe area for more monitoring.

We also have a number of dry contacts in the safe area wired to equipment in the purged enclosure. No problem here because power flowing through the contacts originates from the purged panel.

But what about the relays in the purged panel? Power to the contacts does not originate in the panel but rather at the remote safe area. With the purge lost & door open, even though the local disconnect if off, pulling a relay from its socket could produce an arc from the contacts that are powered.

There could also be the same type of problem in reverse. If I have local relay, whose contacts I use in some circuit, but the coil is powered from the remote safe area. The coil could be under power if it is pulled from its socket.

Is there a standard solution ? The only one I can think of is for the local panel to supply power to the remote panel, both to power the dry contacts and the remote coils. Either directly or to energize a relay in the remote safe panel to enable power to the coils and to the dry contacts.

 

[don_resqcapt19]

I don't really see an issue. Given that for many flammable products you have an IDLH (immediately dangerous to life and health) atmosphere at less than 50% of LEL you can't really be working on the panel if you have vapors in the flammable range.

 

[nakulak]

if you are the plant engineer, then it seems like its pretty much up to you to determine the specific hazards and the safety protocol that errs to the side of caution. As stated above, it wouldn't seem that in a div 2 area someone would be working on it without testing for the hazard and shutting down the area if necessary ? I'm just an electrician so we let other people make these kind of decisions - we just ask the question and document the answer.

 

[KentAT]

But what about the relays in the purged panel? Power to the contacts does not originate in the panel but rather at the remote safe area. With the purge lost & door open, even though the local disconnect if off, pulling a relay from its socket could produce an arc from the contacts that are powered.

There could also be the same type of problem in reverse. If I have local relay, whose contacts I use in some circuit, but the coil is powered from the remote safe area. The coil could be under power if it is pulled from its socket.

Is there a standard solution ? The only one I can think of is for the local panel to supply power to the remote panel, both to power the dry contacts and the remote coils. Either directly or to energize a relay in the remote safe panel to enable power to the coils and to the dry contacts.

Most purged panels have a warning attached that the atmosphere must be tested for a hazardous atmosphere before the panel is opened. You should do this.

kent

 

[KentAT]

Warning as required by NFPA 496:

4.11 Markings.

4.11.1 A permanent marking shall be on the protected enclosure in a prominent location so that it is visible before the protected enclosure can be opened.

4.11.2 The marking required by 4.11.1 shall include the information specified as follows:

(1) The following statement, or an equivalent statement: WARNING ? PRESSURIZED ENCLOSURE ? This enclosure must not be opened unless the area atmosphere is known to be below the ignitible concentration of combustible materials or unless all devices within have been de-energized.
(2) The external area classification for the protected enclosure

(3) The pressurization type (e.g., Type X, Type Y, or Type Z)

(4) The temperature class (T Code) or the operating temperature in degrees Celsius as determined in Section 4.5

Kent