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Transformer Grounding Question

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devriems

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I'm currently working on the control box for my HERMS system and getting ready to wire up the transformer to step the voltage down to 24 for the gas valve. I've listed the transformer and gas valve I'm using below. The installation instructions from Honeywell for the transformer say that it cannot be mounted in a plastic enclosure, only metal. I'm trying to figure out why that is and Honeywell hasn't been much help in getting an answer. My guess is that it is due to one of two things:

1. The transformer can get hot, and so it may melt the plastic container.

2. The transformer does not have a ground wire hookup on it anywhere (primary or secondary side) and so in order to get a circuit to ground they are saying to use a metal box, which would itself be grounded. Since the transformer would be screwed to the metal box, then maybe that is what provides it a path to ground.

I've been trying to figure out if a transformer like this even needs to be grounded and haven't been able to find a definitive answer. I'm using a plastic box for my control panel and don't want to scrap that and start over with a metal one unless it's totally necessary. Anybody have any guidance they can offer?

Transformer:
https://www.supplyhouse.com/Honeywell-AT140A1042-Universal-Mount-Transformer-13620000-p

Gas Valve:
https://www.supplyhouse.com/sh/control/product/~product_id=VR8200A2132
 

day_trippr

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There should be no connection between input or output transformer windings to earth or chassis ground.
The metal enclosure is for safety and it should definitely be grounded...

Cheers!
 
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devriems

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There should be no connection between input or output transformer windings to earth or chassis ground.
The metal enclosure is for safety and it should definitely be grounded...

Cheers!
Thank you for the reply. Yea, if I was using a metal enclosure I would definitely ground it, but I'm using a plastic box and have run the ground wire (coming into the enclosure from the main power) to a barrier strip where I've connected devices that need to be grounded (pumps).

I wonder then why Honeywell states not to install the transformer in a plastic enclosure. If it doesn't need to be grounded, then what could be the reason other than that the transformer could get hot and melt a plastic enclosure (which obviously wouldn't happen with a metal one)?
 

day_trippr

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Well, transformers can run warm depending on how well matched they are to the load, and I suppose if one screwed the pooch and went with a grossly undersized xf it might cause some real issues with a plastic enclosure.

If you can leave your hand in contact with the active transformer core for an indefinite period you're likely fine with the plastic box...

Cheers!
 
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devriems

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I just had a thought. Perhaps they say to only mount it in a metal enclosure due to the weight of it. It isn't a light piece of electrical equipment, and if you put it in a 4x4 plastic box it might bend it a bit. I'll keep pestering Honeywell to see if I can get a direct answer from them.
 

day_trippr

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I doubt that's relevant...

Cheers!
 

lschiavo

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It looks like all the mounting methods suggested would ground the case of the transformer to the metal box or conduit.

Just attach a ground wire to the case somehow.
 

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