Earth leakage/resistance question

[steemy]

Hi. Yesterday I got a call to go look at two, single phase milk dispenser/coolers not running. I arrived to find both units next to each other,both plugged into the same power outlet. I checked for power at the power outlet,230V all ok,then I noticed the digital electronic controls not lighting up. Checked the power on them and they both had 230V on them but nothing happening. So I decided the controls had been fried from a mains power fault. I then got a call from a place 2km down the road with the same problem and I went and looked at that and same thing,controller fried. The chef in the kitchen there mentioned something about a lightning strike.
this is where I would like some info. I checked power to earth which was 140V which is correct here in Norway. So I tried checking resistances to earth with the power on and power off at the single phase power outlets but i wasnt really sure what readings i was looking for. Can anyone help me here? cheers.

[The Viking]

Depends on what meter you used.

Proper "earth leakage meters" or meggers has to be used with everything disconnected, not just turned off.

Ideally on stuff we measure you shouldn't get a reading, it should be open circuit.

But be aware, as these meters are spiking the circuit you are testing, it would have the same effect as a lighting strike on electronic control modules.

[coolstuf]

Insulation should be more than 2 mohms

[Frikkie]

Hello Steemy,

If the voltage in Norway is 230v you should test 230v between the live wire and the earth on a AC volt test. You should also test 0 v between the neutral wire and the earth. This is testing the electric supply at the wall point with the milk cooler unplugged.

I am confused why 140 volt you say should be correct. This can be only if the neutral is floating and I am thinking this could be a supply fault. I have only seen this on boats where the neutral is not bonded with ground because it makes annodic prevention dificult and corrosion of the body of the boat is faster.

Please speak with a Norway electrician because maybe Norway electerical is different and I don't want to give you wrong ideas.

Also a digital tester can be very sensitive and give a wrong impression. It has a input resistance that is too high and it can give confusing readings. I like a needle tester better for this testing.

If there is lightening spikes caused the damage you can maybe replace the small surge arresters on the controller circuit board and it can still be working. I have put in large surge arresters at my main electric supply meter box to stop lightening spikes making damage to my machines. They can be put in easily and cost little to buy.

If you test the machine insulation you should use a megger tester between the metal frame of the milk cooler and the live pin of the plug or at a termination inside the machine. The cooler should be switched on but unplugged. You should use the 250v setting on the megger tester if it has electronic control, like The Viking said high test voltages can spike the electronics. I think Coolstuff is right with a 2 Megaohm minimum allowed.

[Peter_1]

I think you have a loose neutral somewhere on the mains in the street, giving the fact that others encounter the same problem
Steemy, it would be polite and nice if you came back to inform those who helped you.
A simple thank you is more than enough ;-)

[Eeram]

Frikkie is correct. If you're getting only 140 Volts between live and earth, it could also mean that your earthing system is not good.

When you measured between neutral and earth on the controller while the power is on, what was the result?

By the way, what is your 3-phase supply between phases?

[Frikkie]

I think all the answers here could be correct Steemy. We would only know more if you can give us some further information. Some of the faults it could possibly be can be dangerous and might give shocks from the body of the milk cooler so you should please be carefull when testing and also be carefull for your customer. It might be the right time to get a professional electrician to help you make these repairs. It could be nice to know what you find.

Hello Eeram, if he has 230v single phase it would mean 400-420v three phase. There is a nice internet site here (http://users.telenet.be/worldstandards/electricity.htm) with information on overseas supply voltages and frequencies for single phase and 3 phase.

[steemy]

Hi there. sorry for the lag in replying I had a little holiday last week with no internet.
Thanks all for your help. I found out it was power surge from lightning and controllers were replaced and everything works ok.

BUT, the live to earth here in Norway is 130 to 140V. it was one of the first things i was told when i moved up here. However I never asked why.
When I measured between neutral and earth no voltage detected which is correct and to answer the other question 3 phase supply between phases in norway is 240 not 415 like i am used to which is strange.

[steemy]

I just found this on of all places wikipedia. "In North America, a typical three-phase system will have 208 volts between the phases and 120 volts between phase and neutral"

so thats what we have here. But In australia i had 240V to earth and 415V between the phases on 3 phase. can anyone explain the difference?

[steemy]

ahh i misread that quote it says 120V between phase and neutral. Not between phase and earth. Now im very confused.

[Frikkie]

Hello again Steemy,

Hope you had a great holiday, you are making me jelous.

In your first posting you say that the milk coolers were single phase. This means that there would be a single live wire, a neutral wire and an earth wire to make the supply to go to the socket outlet on the wall.

In all the places I have ever seen the neutral and the earth are bonded or tied together electrically at the point of supply, usually in the council supply box where the meter is sitting. This would mean you should always test 0V between the neutral and the earth anywhere in the premises electrical installation.

If the earth bond is poor this voltage might rise when the electrical system is under load. When this happens you might get a much higher reading of this neutral earth voltage with a digital type multimeter than you will with a analogue needle type tester and this can confuse. The reasons for this are complicated and are not really for refridgeration forums I think but it does mean there is a electrical installation fault.

Because the earth and the neutral are bonded the volts readings between the live and earth or the live and neutral should be the same or very close to same.

When the single phase supply voltage is given as 230 volt this is talking about the live to neutral voltage but the live to earth voltage would also be 230volts on your tester. (In USA the single phase supply is 110 volts, this would also be the live to earth voltage.)

If the neutral is not bonded and allowed to have a voltage with respect to earth then you could get very different readings and none of what I have sayd will be correct. I have never seen this system other that in a marine or boat installation.

If you have a three phase supply you have three live supply wires, one neatral and one earth. As before the neutral and earth are bonded and you wil test 0 volts between them. Any one of the lives will test will test 230v to earth or neutral. If you test between any two of the live wires you will see 400-420volts. Some three phase supplies don't have a neutral, so you would have to test to earth in this caes.

In USA the 110v single phase supply means you will test 230volts between any two of the live wires of a three phase supply. This is handy because it means that a 220v single phase machine from South Africa can run in the USA if it is wired onto two of the phases of a three phase supply. Unfortunately this will not work the other way around, a USA machine cannot run in SA without a transformer to reduce the supply voltage first.

The information you have give so far would point to a floating neutral system in Norway but I am very doubtfull this is correct. Please speak to an electrician to find some hard facts then I can maybe help yuo some more.

You need to know what is the single phase voltage (live to neutral). Also is the neutral bonded at 0volts or ground/earth potential? Everything else we can work out from there.

Thank You
Frikkie

[nike123]

ahh i misread that quote it says 120V between phase and neutral. Not between phase and earth. Now im very confused.


In your country power distribution system has IT distribution network (http://en.wikipedia.org/wiki/List_of_countries_with_mains_power_plugs,_voltages_and_frequencies).

International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.
The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):

T : direct connection of a point with earth (Latin: terra);
I : no point is connected with earth (isolation), except perhaps via a high impedance.

The second letter indicates the connection between earth and the electrical device being supplied:

T : direct connection with earth, independent of any other earth connection in the supply system;
N : connection to earth via the supply network.


In an IT network, the distribution system has no connection to earth at all, or it has only a high impedance connection. In such systems, an insulation monitoring device is used to monitor the impedance.
http://upload.wikimedia.org/wikipedia/en/thumb/4/44/IT-earthing.svg/350px-IT-earthing.svg.png




In IT systems, a single insulation fault is unlikely to cause dangerous currents to flow through a human body in contact with earth, because no low-impedance circuit exists for such a current to flow. However, a first insulation fault can effectively turn an IT system into a TN system, and then a second insulation fault can lead to dangerous body currents. Worse, in a multi-phase system, if one of the live conductors made contact with earth, it would cause the other phase cores to rise to the phase-phase voltage relative to earth rather than the phase-neutral voltage. IT systems also experience larger transient overvoltages than other systems.

Check this by measuring both contacts at plug socket against the earth. If both have same voltage against earth, than everything is OK and you don't have neutral wire in your house distribution system.
That mean your power company distribute to you only two phase of 230 V between them, and no neutral wire.
So be careful if that is the case, all switches and disconectors must be two pole.

Also in your country, three phase system is stated at 400V/50 hz which mean it is distributed separate from this 230V supply. As oppose here in my country, where we have combined distribution TN-S system and neutral wire.

http://upload.wikimedia.org/wikipedia/en/thumb/1/13/TN-S-earthing.svg/250px-TN-S-earthing.svg.png

[Frikkie]

If you are in a place where a IT earthing is used then an insulation fault on a phase caused by a machine fault will probably not cause the power to trip. If there is another phase with a low insulation fault then this could cause you to get dangerous voltage on the frame of a machine and give the user a large electric shock. This wiring scheme will also be possible to develope high voltages on a phase under a fault condition which could blow your circuit boards and as Nike123 says you would need to have a intelligent monitor device somewhere on the installation. I have only a little experiance with this system and I am not sure what surge protection is used and what would happen with this system if there is local lightening.

If I was working on machines which are supplied this way I would certainly ask someone with local experiance of this system. The standard tests you read about from countries with other earth arrangements may not be the same for you.