ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

Topic Options
Rate This Topic
#141825 - 10/27/04 10:53 AM Door interlock
lyledunn Offline
Member
Registered: 06/30/02
Posts: 159
Loc: N.Ireland
I was testing a local paper disposal plant that uses Telergon isolators on the three phase motor control centres. Now obviously there is a mechanical interlock ensuring that the panel is de-energised before the door is open. However, after the door was opened I carried out an insulation resistance test on the supply circuit. I noted a low resistance between red and yellow phases. It appears that around 9.6ohms is present between these phases even with the door opened. This was apparent on every Telergon isolator in the plant. I had to disconnect the supply cables to obtain a reading. Because this was a relatively
pressured job, I really didnt have the time to assuage my curiosity! Any ideas?
_________________________
regards

lyle dunn
Top
Test Equipment:
Large Selection of Test Equipment For Electrical, HVAC, Test & Measurement
Large Selection of Test Equipment For Electrical, HVAC, Test & Measurement
#141826 - 10/27/04 11:28 AM Re: Door interlock
pauluk Offline
Member
Registered: 08/11/01
Posts: 7520
Loc: Norfolk, England
Industrial stuff like this is a little out of my league. It sounds very odd though, as that sort of resistance value would equate to 40-ish amps at 415V.

What's the rating of the fuse/CB feeding each cabinet? Could it be some sort of extra belt-and-braces safety circuit, intended to trip out the supply if it should somehow become energized even though the door is opened?

(Microwave ovens often have an extra door-interlock switch which puts a short on the supply downstream of the main switch, just in case it fails.)
Top
#141827 - 10/28/04 02:51 AM Re: Door interlock
lyledunn Offline
Member
Registered: 06/30/02
Posts: 159
Loc: N.Ireland
Paul,
The fuse protection was 200 to 630 amps. I concur that this may be associated with an additional electrical interlock, I never got the opportunity to explore as the plant manager was dancing every time a machine was down! Dont forget the 9.6 ohms was a resistance value which may have simply been the resistive part of a coil.
_________________________
regards

lyle dunn
Top
#141828 - 10/28/04 03:42 PM Re: Door interlock
pauluk Offline
Member
Registered: 08/11/01
Posts: 7520
Loc: Norfolk, England
Actually you saying "coil" has made be think of something else. Is there any chance there could be a solenoid interlock on the doors, so that they cannot be opened when power is applied?

Even though there will be inductance present, under 10 ohms DC resistance still seems pretty low for a coil used in such an application though, so maybe that idea's way off base.
Top
#141829 - 10/31/04 05:57 PM Re: Door interlock
Trumpy Offline


Member
Registered: 07/05/02
Posts: 8211
Loc: SI,New Zealand
Lyle,
Just a little question.
Are there no control transformers across the phases to step down to a lower voltage?.
But as Paul also pointed out, at 9.6 ohms, it's going to be pulling some serious current.
400/24V transformers are pretty common here for control purposes.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin
Top

Member Spotlight
Member Since: 12/24/00
Posts: 4259
New in the Gallery:
SE cable question
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box


Who's Online
0 registered (), 55 Guests and 12 Spiders online.
Key: Admin, Global Mod, Mod
 
New in the Gallery:
SE cable question
 
Top Posters (30 Days)
Admin 47
HotLine1 43
gfretwell 19
Ruben Rocha 12
Trumpy 9
 
Newest Members
Scotto, Freecrowder, clee512, Jdscott2005, FAIZAN

ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals