Earth Leakage

[govender1]

Could you explain the operation of a domestic earth leakage unit. Can lights be connect to earth leakage? Will you get nuisance tripping when a light bulb pops?

Thanks

 

[fb599]

I think you are referring to RCD protection here. Generally RCD protection is fitted to your socket outlets and ring/ radial circuits.(30mA for sockets, 100mA for fixed equipment. fridges etc) The last thing you want is a 40W lamp taking a whole installation off. It all depends if you are on a PME system too. Normally your lighting circuits would not be protected by RCD protection. If you are in the UK, the IEE wiring regulations will help alot. Try a library if necessary, there are many books outlining the principles of earthing. Finally ask your local utility if they have a guidance document of supply arrangements. Hope this helps.....

 

[gce1991]

Hi

The operation of Domestic Earth leakage is somethig like this.

It is done basically by a device called RCCB ( Residual current circuit Breaker). It basically detects the leakage of current interms of milliamps. Standard is 30mA, 100mA and 300mA. Note that 30mA is more sensitive than other.

It basically has a current transformer sensing the phase and neutral side. Normally operation ( No fault), the summation of current flow through neutral is zero and when there is a leakage, it sense the current through the current transformer and opens the breaker thereby isolating the supply and load side

Hope this helps
soma(Electrical) G.P.A.Eswaran
Project Engineer
QuEST-GE GEDC
e-mail: arthanarieswaran.g.p@quest-global.com

 

[electricuwe]

Using such unit in domestic installations can be very helpful in protecting people from electric shock hazard (only the 30 mA devices are suitable for this purpose)if electric equipment fails. It will also reduce the risk of fire if defective equipment causes current to flow on unintended earth connections.

But do notconnect devices like fridges to circuits protected this way unless it is required by standards.

You may find a lot of rotten food at home after nuisance tripping when you come back from a holiday.

If you also want to protect these devices use a separate RCCB device for these circuits.

 

[Graeme38]

An earth leakage device (GFI) or RCD in Europe, measures diffential current between the phase and return conductor. The difference in the current flowing down the earth conductor.

In the Uk we do not connect to lighting circuits, more from a point of safety.

Nusiance tripping can occur due to transients etc. it depends on the manufacturing standard. Most modern RCD's incorporate filter circuits to limit this problem. Light bulbs blowing do produce transients that can cause tripping.

 

[peebee]

The short answer is: avoid putting anything much more than absolutely required on the GFI (RCD) device.

The long answer is:

1. I made the mistake of wiring a fluorescent fixture in my kitchen to a GFI outlet. Flipping the switch on will trip the outlet once in a while. It never trips after it has started, only instantaneously with starting. This happens only about once every month or two, not every day. I cannot explain why the starting current would not be equal on the phase and neutral, but apparently there is some small difference there about equal to the trip level of the receptacle.

2. Every device has some small associated ground leakage current. Every device you add to a GFI circuit will add to the total ground leakage seen by that circuit. It does not take too many downstream devices leaking at maybe 3 to 10mA to add up to 30mA, and start causing a bunch of nuisance tripping. This effect is exacerbated in areas where water can condense, such as cold rooms, bathrooms and outdoors (generally, most of the places where you are required to provide GFI).

 

[Bung]

Nobody seems to have covered how the thing works yet, so here goes. The basic concept is that the earth and neutral are connected together (this is sometimes known as an MEN point)upstream of the RCD, in your distribution board. The phases and one wire (used as the neutral) from one side of the MEN point are taken through a core balance CT. This CT is coupled with a protective device set to trip at whatever the allowable leakage is (typically 30mA in domestic situation). The earth wire into the house also comes from the MEN point (the other side of the MEN link in practice).

If there is a fault between active and earth in the house, some current returns in the earth wire and so reduces the current in the neutral wire, so causing an imbalance in the core balance CT - and hey presto, the protective device trips.

Bung

 

[Graeme38]

Following on from peebee, for UK wiring regulations we are required to test the tripping current and time of an RCD(GFI). Generally speaking this can vary between 19mA - 26mA on a functioning device. I also have to say that I have found some new from the manufacturer not to function at all.

Standard UK practice is to limit the earth leakage on any circuit to 25% of the nominal rating of the RCD. Typical IT equipment can produce as much as 3.5mA of earth leakage, so where you may have a PC and screen and UPS it is possible in theory to have as much as 10.5mA of earth leakage.

Where earth leakage on any circuit may in operation exceed 10mA we also have to take additional steps for safety in teh form of running an additional earth wire etc.