Earth leakage tripping with motor inverter. Please help

15 Nov 2012
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United Kingdom

I have a problem since I fitted an inverter in my workshop to run a metalwork lathe.

It is a filtered inverter which supplies three phase power to the motor. Since it was fitted, the inverter intermittently trips the main earth leakage trip in the house.

Sometimes it will trip as soon as it is turned on, but a couple of days a go it tripped after being left on (but not being used) for about 2 hours. Resetting the trip whilst the inverter was still in the on position, caused the trip to go after a few seconds. only turning the inverter off would cure the tripping. Another time, it tripped when I started the lathe. I am convinced the inverter must b e the problem, but do not know how to test it (it was brand new, but from ebay so not guaranteed).

I had a 2hp inverter originally (this was new from IMO) which simply would not work at all, tripping at switch on every time. The makers said the only was to solve it was to remove the internal filter, as the leakage was over the (I think), 30mA of the trip.

I was told then the the size now fitted 1hp would not give any problems.

What I wanted to know is if I can fit a separate earth leakage trip just feeding the inverter, that perhaps would have a lower trip rating as the main one, or perhaps the same rating but it might trip quicker, if fitted close to the input to the inverter, so if it does trip, it won't cut power to the rest of the house.

Also does the type of current trip (i.e. b class .etc) make any diffrence, since it is only in line with the mains and obviously not connected to the earth, only to the live input to the inverter.

Please note you are not replying to an electrician but I am competent to wire electrics, and never had a problem before this one.

Any help would be greatly appreciated.
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1) Not all RCD's are the same if you read this on X-Pole it would seem they trip at 90% of 30ma not 50% and also give some indication when it's approaching the trip point.

2) Using a RCBO the device will only do that circuit not all so if there are multi items all adding up to cause it to trip then this may cure the problem.

3) An outbuilding (not sure if workshop in house or not) would not normally be feed through the house RCD and would have it's own RCD.

4) For a fixed appliance feed with cable complying with BS 5467, BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436 or surface then there is no need for RCD protection. Depending on earth arrangement it may need a RCD but not 30ma. Even with a plug and socket as long as the socket is marked for use with only one named item then you don't need a RCD. So with a SWA feed cable you can do away with the existing RCD.

There are many consumer units. Some have all RCBO's and no common RCD, Some have just 2 RCD's, and some have 2 RCD's and some RCBO's as well. But in England and Wales to change the consumer unit is not cost effective as DIY due to Part P. The same applies to adding a Henley block and a mini Consumer Unit.

Because the Consumer Unit is a type tested distribution unit you can only fit RCD and RCBO and MCB of the same make even if other will fit.

So although there are ways around the problem unlikely to be a DIY job. The same applies to using an isolation transformer as used with boat supplies to fit one is not straight forward and not really a DIY job.
Thank you very much for kindly writing back but I have to say most of it was not understandable to me. What I mean by that is that you are talking electricians speak with too many initialised words for things that I don't know what they are.

I did read the link you put, and got from that, that there are trips that have anti-surge properties similar to a time-delay fuse. Are you saying I need to get one of those to replace the one in the consumer unit ?

Or, are you saying I can fit one in-line with the supply just before the inverter.

Does the "type" of current trip matter as well ? Can the wrong type cause the earth leakage trip to trip ?

Thank you.
If you fit an isolating transformer before the inverter it will stop the nuisance tripping.
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To be blunt what ever method is used likely you will need an electrician to do it.

The numbers mean a type of wire which if you put a nail through it will always cause the fuse or MCB to blow or trip.

But what I have said my be of interest but end of day you need an electrician.

As repeated you can use an isolation transformer but the earthing then becomes complex not a DIY job.

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