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Posted By: Belgian Interesting Fault - 05/20/05 10:38 AM
Today I came across an interesting fault and I havnt figured out yet what it could be. Any advice or ideas greatly appreciated.

The main breaker from the poco is 63A. I have a panel monofase (1 fase+N). There are 2 RCDs. The main one is 300mA which covers the whole panel and the 2nd one is 30mA which covers bathrooms, washing machine and central heating boiler (which rund on gas).

The main breaker from the poco is on. When I measure between fase and Neutral I get 0 Volt, between fase and earth=224V, between Neutral and earth 224V! The Neutral is not broken.
If I push the test button on the main RCD 300mA, then it jumps.
When I disconnect the 2nd RCD 30mA, then all the other circuits work fine.
I measure between fase and Neutral I get 224 Volt, between fase and earth=224V, between Neutral and earth 0V (as it should be).
When I reconnect, the problem comes back.
After having reconnected I turn off all the breakers which are under RCD30mA, turn on the RCD and turn all the breakers back on and low and behold, everything works fine!

There are 2 things which maybe I should mention:
1, Both RCDs are 40A whereas the main breaker is 63A, so they should have been 63A as well.
2, There were different issues with the central heating boiler (which is under RCD 30mA).
Could it have to do with the boiler?

For me it's a riddle. If any of you has any logical clear explanation, please let me know.
Posted By: Texas_Ranger Re: Interesting Fault - 05/20/05 02:54 PM
To me that sounds like the Neutral has somehow become a phase... and since it's the same phase you measure 224V to ground but 0V between the wires. How that could have happened... sounds like it's time for an isolation resistance test.
Posted By: Belgian Re: Interesting Fault - 05/20/05 03:54 PM
If that would be the case, then it would have to make a short and the poco breaker should switch off. But it didn't.
Posted By: C-H Re: Interesting Fault - 05/20/05 03:56 PM
Is this a TT system? Has the earth wire somehow been disconnected from the earth electrode? An phase-earth leak (or direct short circuit) after the 30 mA RCD would not be noticed by the RCD if there is no connection to the earth electrode. It would go away when you switch off this RCD or the breakers under it. It sounds odd though, as one would expect some connection through appliances.
Posted By: Belgian Re: Interesting Fault - 05/20/05 05:22 PM
Yes, C-H it's a TT system. The earth wire was very well connected to the earth in the panel as well as in the cellar, where it's connected to the earth electrode! This building was built in 1998, so all sockets have a earth connection and all connections are neat and tidy according to the actual norms.
Posted By: gideonr Re: Interesting Fault - 05/20/05 07:12 PM
Are these double pole breakers? Where are you reading the voltages?
Posted By: Belgian Re: Interesting Fault - 05/21/05 09:47 PM
Yes, Gideonr, they are double pole breakers. The voltage was measured after the PoCos breaker and after the first RCD 300mA
Posted By: Trumpy Re: Interesting Fault - 05/22/05 09:32 AM
Eli,
It sounds to me like Phase and Neutral have been transposed (swapped).
Call the PoCo.
Posted By: Belgian Re: Interesting Fault - 05/22/05 05:25 PM
No, They were'nt transposed because when I reconnected, then the readings were correct. Between Neutral and earth= 0 V , Between phase and earth 230V. It is a very strange story. The meter which I use is a Fluke, so I wouldn't say that its a meter fault.

[This message has been edited by Belgian (edited 05-22-2005).]
Posted By: gideonr Re: Interesting Fault - 05/22/05 09:54 PM
My feeling is that one of the breakers may be faulty, possibly one of the poles gone, or its not snapping open. The oddness of it all suggests there may be more than one fault here. Are you quite sure about the earth integrity, e.g. are you getting volts to the boiler pipework?
Posted By: djk Re: Interesting Fault - 05/22/05 11:25 PM
Surely if the voltages were read after the poco breaker i.e. just after it. It's a problem on their end ?
Posted By: chipmunk Re: Interesting Fault - 05/23/05 12:56 AM
I agree DJK, sounds like a loose neutral on the PoCo's breaker, or maybe intermittent contacts on it. I am assuming their breaker is a standard type, non-rcd, thus it wouldn't mind the loss of the neutral, and maybe the disturbance caused by switching, testing etc is intermittently connecting and disconnecting whatever fault it is. Very mysterious, and lost neutrals always make me highly nervous, people assume no power=dead.
Posted By: Belgian Re: Interesting Fault - 05/23/05 10:13 AM
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My feeling is that one of the breakers may be faulty

Gideonr, my feeling says the same, but I yet have to find a logical explanation.

Djk, When I disconnected all the the circuits which are after the PoCos breaker, then the fault went away, so I dont think that its a fault by the PoCo.

Chipmunk, we are talking here about a appartment of a building, not a house. The PoCo breaker is in the cellar and the panel is 3 floors higher. So there is no mechanical influence (to PoCos breaker) when touching the breakers 3 floors higher.
The problem was only in one appartment. The other floor didnt encounter any faults. Concerning the loose neutral on the PoCos breaker, well ,that wasone of my first things to control, and since it's a recent built building, its very easy to see through the meter panel.

Lets put it this way :

Question 1:
How can a healthy monofase circuit fall from 230V to 0V (between phase and N) only when the Neutral of a load is connected (after that phase is already connected) and without tripping a 63A breaker?
Can a major fault be enough to bring down the voltage to 0v and not enough to trip the breaker?

Question 2:
How can a neutral have a 230V reading between neutral and earth only when connecting the Neutral (of the load)?




[This message has been edited by Belgian (edited 05-23-2005).]
Posted By: chipmunk Re: Interesting Fault - 05/23/05 11:07 AM
Ahh, it's making more sense now. I wasn't aware of the distance between breakers.
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Question 1:
How can a healthy monofase circuit fall from 230V to 0V (between phase and N) only when the Neutral of a load is connected (after that phase is already connected) and without tripping a 63A breaker?
Can a major fault be enough to bring down the voltage to 0v and not enough to trip the breaker?
A very high resistance contact on the neutral somewhere between the PoCo breaker and the point you're measuring the voltage would do it. Voltage looks normal with no load, but then goes to 230 on the neutral when you put a load on
Quote

How can a neutral have a 230V reading between neutral and earth only when connecting the Neutral (of the load)?
I am still not 100% clear on the wiring, but it sounds like it's being affected by the same fault.

Is there any possibility that the wiring has been damaged between the floors, such that only the one apartment would be experiencing the fault? Small holes in the insulation of the cable + damp or condensation have been known to do things like this.
Posted By: Belgian Re: Interesting Fault - 05/23/05 02:19 PM
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Voltage looks normal with no load, but then goes to 230 on the neutral when you put a load on

What do you mean "goes to 230 on the Neutral"? Measuring Neutral to earth?

[quote]Is there any possibility that the wiring has been damaged between the floors, such that only the one apartment would be experiencing the fault?[quote]

It's very unlikely that the wiring has been damaged. We are talking here about a cable XVB 3G10 which is 10mm2, so it's also unlikely that there is a high resistance.
Posted By: chipmunk Re: Interesting Fault - 05/23/05 08:42 PM
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What do you mean "goes to 230 on the Neutral"? Measuring Neutral to earth?
Yes, it appears from your description that when you turn any load on, the neutral is rising to phase voltage with reference to earth. I agree, this does seem a very interesting fault, you have eliminated most of the possibilities in your testing.
Posted By: gideonr Re: Interesting Fault - 05/23/05 11:18 PM
So what your saying is, the breaker (and I'm not really sure which one) is fine if turned on with no load and then loaded, but fails in the neutral pole if turned on with a load connected?

Relays can do this, which is why their contact material has to be chosen carefully for the application. With relays, the contacts bounce and spark, and the spark puts some oxide in between the contacts.

In reply to your two questions, if the current pulled by the "load" to earth is under 30mA/300mA they will not trip, and clearly there appears to be no amps overload. A bad neutral would be dragged up to phase voltage by the load.


[This message has been edited by gideonr (edited 05-23-2005).]
Posted By: pauluk Re: Interesting Fault - 05/24/05 08:18 AM
I agree with the previous diagnosis that the neutral is bad somewhere along the line. It might be a bad connection in the main breaker, or possibly intermittent contacts on it.

At the times when there's a relatively high resistance through the neutral side due to the fault, you'll measure 230V phase-to-neutral and 0V neutral-to-earth with no load because the meter is pulling a tiny current.

As soon as you connect a substantial load, the resistance of the fault becomes significant and on the load side of the fault you'll read 230V neutral-to-earth due to the circuit completed through the loads.

There's no reason for the RCDs to trip, as the small current on their two poles will still be equal.

If you could temporarily disconnect the feeder from the load side of the incoming main breaker and wire up a suitable load at that point, you could determine if the fault is on or before the main breaker or somewhere further down the line.


[This message has been edited by pauluk (edited 05-24-2005).]
Posted By: kiwi Re: Interesting Fault - 05/24/05 08:57 AM
I think Chipmunks got it. Looks like a bad neutral contact somewhere, and maybe another earth - neutral fault somewhere on top of that.

Maybe one of the RCDs( under-rated at 40A ) has over-heated and done something weird.

Sounds like there is a possibility of some current flowing in the earth. Have you checked this ? For that reason I guess this
really needs to be sorted out.

Please let us know how you get on with this problem Belgian, because we're all dying to know the outcome.
Posted By: Belgian Re: Interesting Fault - 05/24/05 05:17 PM
My first thought was like you all point out that its a bad neutral connection, somewhere between the cellar and the third floor.
Let me explain to you why I dont think that its a neutral fault:
When I disconnected the Neutral feeding the rcd 30mA, then the fault went away. i.e. 230v phase to N and 0v N to Earth.
This RCD is only part of the panel (on 3rd floor), which means that when I disconnected it the rest of the panel was connected with all the loads on it. So the fault is clearly somewhere after the RCD 30mA and not in the Neutral from the cellar to 3rd floor.

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Maybe one of the RCDs( under-rated at 40A ) has over-heated and done something weird.
That's possible, but how does it explain this fault even if the RCD30mA is burnt?


Quote
Please let us know how you get on with this problem Belgian, because we're all dying to know the outcome.

The thing is that this problem isn't there anymore after I turned of all the breakers which are placed ater the RCD 30mA and switched it back on. Firthermore after having explained to this client that they would need to change both RCDs since they are under rated, after having heard the price of both rcds 63A, the client said that they will "have to think it over", which most probably means that since everything is working now, they dont want to make any expenses.


[This message has been edited by Belgian (edited 05-24-2005).]
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