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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> floating earth on an inverter! https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1258402438 Message started by watte on Nov 16^th, 2009, 8:13pm

Title: floating earth on an inverter!
Post by watte on Nov 16^th, 2009, 8:13pm

Hi Everyone,

The other day i bought an inverter to run from 12v dc 60Ah battery, output 230v ac. When i had it running i recieved a shock from a piece of computer equipment connected to the output. Having looked closely again at the instructions, a small paragraph in the book told me to link the neutral and earth together inside the unit to the case of the inverter, as it has afloating earth. Before i did this i checked the output voltage referenced to the case and found 115v on the live and neutral. After i tied neutral and ground together, zero volts was present on the neutral line. The inverter is primarlly used on marine applications, but in this case, it is used on land, to power up a host of computer items, incase of power failure. I emailed the inverter supplier in the uk, and i'm still waiting for reply, and i will probably carry on waiting!

So can anyone tell me what is a floating earth, and how tieing earth and neutral together, and to the case can creates a proper earth?

Also, if i now fit an rcd to the output would it trip correctly because of the above?

Still can't work out why the client has to link the wires, surely it should be done at the factory, this was not a cheap piece of equipment?

P.S pat tested all computer items, all ok.

Thanks for taking the time to look. ::)

Title: Re: floating earth on an inverter!
Post by LS on Nov 16^th, 2009, 10:20pm

If it's floating then it's not connected at a reliable earth potential and it may be segregated to stop interference.

You need to be very careful when using inverters, you need a reliable earth connection if using class 1 equipment with them, even more so with computers as these leak current to earth as normal, which creates an immediate shock hazard. You also run the risk of damaging your IT equipment if you do not allow for a suitable earth connection.

I'm not big on inverters and batteries, it would be sensible to ground the earth onto the neutral for the time being, but I'm not sure reliable this is as a path to earth, not very I'd imagine.

You don't mention much detail on where this would be used on land, if it's in an office type environment then normally you would buy a purpose built UPS device, which would then rely on the fixed wiring earth connection even when the power fails.

Title: Re: floating earth on an inverter!
Post by CWatters on Nov 17^th, 2009, 8:20am

What does the manual for the computer equipment say? Desktop computers tend to need an earth - I mean a proper connection to planet earth provided through the power supply wiring/mains plug. An inverter/car battery can't provide one on their own although some inverters have a screw terminal on the case so you can connect an earth wire to a metal stake hammered into the ground (or the earth system of a metal ship or the building earth?). Laptop computers and their chargers maybe double insulated and not need an earth? Check what the all equipment requires.

How exactly did you get a shock? What did you touch? Without an earth the case of the computer may float to who knows what voltage. That may give you an interesting tingle BUT it should still be isolated from the mains so the current should be low. Not safe though as you aren't protected from faults.