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Topic Title: UPS earthing and hazardous areas
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Created On: 27 September 2017 10:27 PM
Status: Read Only
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 UPS earthing and hazardous areas   - electronicsuk - 27 September 2017 10:27 PM  
 UPS earthing and hazardous areas   - AJJewsbury - 28 September 2017 10:24 AM  
 UPS earthing and hazardous areas   - electronicsuk - 28 September 2017 12:54 PM  
 UPS earthing and hazardous areas   - AJJewsbury - 28 September 2017 07:22 PM  
 UPS earthing and hazardous areas   - gkenyon - 29 September 2017 02:36 PM  
 UPS earthing and hazardous areas   - kellyselectric - 01 October 2017 03:59 PM  
 UPS earthing and hazardous areas   - electronicsuk - 01 October 2017 05:24 PM  
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 27 September 2017 10:27 PM
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electronicsuk

Posts: 4
Joined: 25 September 2017

There are plenty of topics here about UPS earthing arrangements, but I don't think that one has been covered before. On our site we have a gas chromatograph that was recently damaged due to a power quality issue. I've suggested that we put it behind a UPS.

In theory it should be simple; it's a small load, about 100w, and it's the only load that would be fed from the UPS, so could conceivably be considered electrical separation (413.1.2). My understanding is that most modern UPS systems rely on the incoming supply to provide some reference between N and E, and if both poles of the incoming supply are disconnected, the load effectively has a floating earth.

I wouldn't normally be too concerned as small UPS systems are obviously designed by the manufacturer to operate with a floating earth on the output when running off battery. However, there's an extra layer of complication here as the gas chromatograph is an ATEX rated piece of equipment in a potentially explosive atmosphere. BS EN 60079-14 and -17 don't make any specific reference to UPS systems that I can see, nor does my CompEx guide book.

Maybe I'm just being over-cautious, but the idea of a floating earth in such an installation just doesn't seem right. Playing devil's advocate, if supply L & N were isolated at an upstream DB somewhere in the installation, the UPS would take over. As the N has been disconnected, there's no longer any current path between N & E as far as the UPS is concerned, as it relies on the two being connected at the substation. In an ideal world the load on the UPS would now be electrically separated, a fault between L & E should have no current path, no spark can occur and an explosive atmosphere can't be ignited. In the real world, certainly if the UPS maintains a connection between supply and load N when on battery, as some do, I'm not at all confident in such a scheme. Even capacitance between N and E of the supply cables (up to the point of isolation) could allow enough current flow to cause a spark in the event of a L-E fault at the load.

The best solution I have so far is to put a 1:1 isolation transformer between the UPS output and the load, linking N-E on the transformer's output, effectively creating a small TN-S installation. This 'new' earth would also be connected to the incoming supply earth to ensure no potential between the two. Even better would be to find a UPS with a built-in isolation transformer, but the only ones I can find are 120V American models.

Would appreciate any advice or pointers, especially from anyone with experience fitting UPS supplies to equipment in hazardous areas.
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