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Topic Title: Crabree grid switches and metal halide loads
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Created On: 04 December 2009 08:02 PM
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 04 December 2009 08:02 PM
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hibby

Posts: 48
Joined: 02 March 2005

A Job I've been on today in a shop had a problem Switching 150W metal halide fittings (Magnetic Gear) from 20AX single pole Crabtree grid switches. Contacts are welding. This is occuring on circuits with either one or three fittings connected. Two replacement switch modules have suffered the same fate. Anyone else had issues with switching inductive metal halide loads with these modules ? Short of contactoring the lot (many circuits), can anyone suggest any switches better able to handle the inrush ?
 07 December 2009 05:41 PM
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AJJewsbury

Posts: 10964
Joined: 13 August 2003

How many 150W fittings on a switch?
- Andy.
 07 December 2009 06:15 PM
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JonSteward

Posts: 560
Joined: 04 December 2007

I've seen this problem with time clocks where the contacts weld. Mains filters were required to be connected across the terminals when using highly inductive loads.
 07 December 2009 10:38 PM
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hibby

Posts: 48
Joined: 02 March 2005

Hi Andy,
There are 4 lengths of 3 circuit track with 12 switches. Each switch controls one circuit of 1 track. Each switch has anywhere between one and three fittings connected (mainly 1 or 2). The problem is occuring on switches with only one fitting connected as well as those with two or three. Each track section is on a separate MCB (4 circuits)
More often than not loostening the grubscrew to remove the switch module from the grid, frees the contacts - for a while.
I was going to try some MK grid plus 20AX modules and see if these perform any better.
 08 December 2009 07:19 AM
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JonSteward

Posts: 560
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Does sound like tje switches are distorted in their grid. How old are the lamps in the fittings?
 08 December 2009 03:27 PM
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hibby

Posts: 48
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The affected switches are spread over 4 separate grid frames in separate mounting boxes, so I don't think they are distorted. The Lamps (and fittings) are about 3 months old with around 200 hours on the clock. Another switch module failed closed yesterday, and my colleague cut it open. The contacts could be pulled apart by mild force, but were definately welded. I've now got some MK grid plus modules and frames to try later in the week.....
 09 December 2009 01:19 PM
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AJJewsbury

Posts: 10964
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Each switch has anywhere between one and three fittings connected (mainly 1 or 2).

That kills my theory then (I was hoping you'd say dozens and dozens so I could say divide them up!)

I've not that much experience of MH fittings, but it seems odd that that many (or rather few) fittings would cause enough inrush to upset a 20A switch - especially an X rated one.

I guess a snubber network across the switch could help, but I can't help thinking it shouldn't be necessary (and I would worry about the switches ability to operate as an isolator).

I'm guessing more along the lines of some other cause at the moment - distorted when fitted (like Jon said) or a bad batch of switches perhaps. Others may guess better though.

- Andy.
 09 December 2009 04:46 PM
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broadgage

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I would expect the switches to handle the load without problem.

Perhaps the switches are fakes, and not in fact made by the firm claimed ? I have certainly heard of fake MCBs and socket outlets, so why not grid switches ?

Might be worth sending a sample to the manufactuerer and asking for there comments.
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