Earthing an Inverter to Bus Bar in Garage Consumer Unit


Postby cinn » Wed Jan 07, 2015 11:03 am

I am in the process of converting a garage into a games room with a dual power supply. There is an existing mains supply with an old consumer unit in the garage. This is connected to a 17th edition unit in the main house as well. Additional to this, I have installed an off-grid photovoltaic system to the garage roof and installed dedicated power outlets from a 5kW inverter. At present this system is not earthed which, as it is providing 230 v AC, it probably should be. My question is: can I connect the grounding point of the inverter to the earth bus bar in the garage consumer unit? If so, should I use earth wire or 2.5mm wire? Or, should I purchase a grounding stake and clamp? Again, earth wire or 2.5mm connection?
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Postby ericmark » Fri Jan 09, 2015 10:07 am

Question one has to be what is the supply type. If TT then no problem if TN-S then very little problem if TN-C-S then there is quite a problem.

The problem is under fault conditions neither do you want your earth rod to be the earth for the street nor do you want a fault to leave you without an earth.

Because you need to bond the earth rod will end up being connected to the DNO earth unless using a TT system.

You will read that you can’t fit an earth electrode to a TN system. This does not mean you can’t connect an earth rod it just means it’s not called an earth electrode it becomes an extraneous-conductive-part.

The problem with a TN-C-S supply is should the combined neutral and earth get damaged in for example road works hitting the cable then the neutral will be any metal bonded which is earthed. So where all houses in the street have metal gas and water pipes it is shared between all and no single connection has excessive current. However where all around you have plastic water and gas pipes then in that situation all your neighbours will try getting their neutral from your earth rod.

It would depend how good the earth rod is and at 60Ω not enough current will run to cause a problem. At 2Ω however over 100A could run to your earth rod. Likely an earth rod would be around the 60Ω mark.

Not fitting an earth rod would mean if road works for example completely cut the cable then you have no earth.

So correct answer is:- If not already TT then make it TT.

However in real terms chance of road works either causing loss of neutral or loss of earth are slim. So the person doing the work has to do a risk assessment and decide which way to go. Personally I would stick in an earth rod measure it and then decide if I am going to retain the DNO earth or not.

As to cable size no earth wire which is not part of a cable it will never be less than 4mm² there is a list of minimum sizes which vary according to supply type. Since I don’t know supply type I can’t advise on size. But likely 6mm² will be good enough.

This is clearly not a DIY job. To measure an earth electrode there are two methods likely easy way is to compare to DNO earth using a loop impedance meter. These are expensive to hire and likely cheaper to get an electrician to measure for you.

Remember likely RCD protection is also required and with simulated sine wave inverters you can’t use type AC need type A or B.

Also many inverters are internally connected neutral to earth but not all and one has to be very careful.

I got caught out with a generator where the manufacturer (Honda) instead of connecting earth to neutral connected it to a tapping 55 volts from neutral so earth neutral caused a short.

Some inverters are designed to run with a IT system like the bathroom shaver point which is OK when supplying a single device but not with multi devices.

To me this is not a DIY job there are far to many risks to assess.
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