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Topic Title: Earth Leakage test on 3 phase appliance
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Created On: 02 November 2013 09:14 PM
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 02 November 2013 09:14 PM
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timjames2

Posts: 118
Joined: 17 January 2006

Hi Guys,

I'm increasingly being asked about 3 phase appliances, and the normal way to PAT Test them is to use a simple adaptor with the earth connected, and the 3 lives (and neutral if there is one) shorted out in the adaptor. This allows Earth Bond and Insulation Tests to be carried out. Sometimes (often) the appliance has a contactor circuit, or electronic switch which means it's not all checked by the normal insulation test, and an earth leakage test would be more appropriate.

I know Seaward make a special adaptor box incorporating a current transformer, but this can only be used with their testers. I'm thinking of making up a short adaptor lead with 4 or 5 single cables, then using a current clamp around the earth wire to measure the leakage current whilst the appliance is in operation.

Is this considered good practice, is it what people already do, or am I missing something obvious? I've used the technique on meter tails before (measure the difference between L + N) and also on the earth wire in the meter cupboard to measure earth leakage on a domestic installation. I don't see any reason why it shouldn't work on 3 phase appliances too. Is it regarded as a bit 'heath robinson' or can I recommend the technique to people on my PAT courses?

-------------------------
Tim James
PAT Testing Expert Ltd

PAT Testing Course
 03 November 2013 12:03 PM
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DOUGIE1000

Posts: 4163
Joined: 13 August 2005

is it not just a sticker exercise??

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 03 November 2013 12:09 PM
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MrP

Posts: 835
Joined: 24 March 2006

Pat testing is one of lifes great cons

MrP
 03 November 2013 01:11 PM
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AdrianWint

Posts: 262
Joined: 25 May 2006

PAT testing three phase kit is always a tricky one. Some argue that its outside the scope of the PAT guidelines (I guess partially backed up by the fact that nobody makes a three phase PAT testing box).

The solution I came up with is sort of along the lines you are thinking..... joining the three phases & neutral together so that you can test between these & earth doesn't really prove a lot because you cant make the equipment 'live' in the true sense. I found an earth leakage trip module that used a core balance CT to detect the difference between the vector sum of the three phases & neutral, the control unit of which also had a real time digital readout of the actual leakage. I've built this up into a box with a 32A 3P+N+E inlet & outlet and use this with a selection of leads.
 03 November 2013 01:59 PM
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daveparry1

Posts: 6153
Joined: 04 July 2007

I think you've over complicated the task many times over there Adrian!

Dave.
 03 November 2013 02:19 PM
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alancapon

Posts: 5759
Joined: 27 December 2005

Joining the three phase conductors may have an advantage. If (for example) you had a leakage from each of the heating elements in a three phase device of 0.5A, you would see zero in your core balance CT, as the earth leakage would cancel out within the device. If you took the same device with the three phases connected together, you would see 1.5A of earth leakage. I am not advocating attempting to run a three phase device from a single phase supply, just using this as an example.

Trying to test every part of a device using a "live test" may be near impossible for a Portable Appliance Test. Taking a washing machine as an example, you may need several hours to test it, as several programs may need running so that every heater, motor, level detector and thermostat has been energised during the test.

Regards,

Alan.
 04 November 2013 01:33 PM
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AJJewsbury

Posts: 11349
Joined: 13 August 2003

Sometimes (often) the appliance has a contactor circuit, or electronic switch which means it's not all checked by the normal insulation test

Although unless the contactor is DP, the 'controlled' component will usually still be connected to one pole of the supply (e.g. N) - so PE - L&N (or L&L&L&N) test should still spot a problem.
- Andy.
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