"Live" neutral and earth wires


Postby marekbzikot » Mon Nov 17, 2008 4:14 pm

Hi,
I have a garage and have a consumer unit in it. It is fed by a 6mm armoured cable with live, neutral and earth. Each circuit has a a trip switch and the whole thing is protected by a rcbo.
My problem is that I have a circuit which has neutral and earth live!! The problem doen't stop there. If I connect the + from a seperate cable to the + from the supply and have all the wires seperated then they all become live EVEN though none of them are connected, touching etc. It doesn't matter what cable I use so there is no issue here about a damaged cable. All the wires are insulated with in the insulated cable including the earth wire. The rcb etc does not trip. Since this is a garage and there is a lack of metal pipes do I need an earth spike outside, and is this an earthing issue or have I defied the laws of physics!! The garage is 50m from the housewhere the supply is taken from.
Thank you in advance for your help
Marek
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Posts: 4
Joined: Mon Nov 17, 2008 3:57 pm

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Postby ericmark » Mon Nov 17, 2008 10:11 pm

How have you tested? I would expect to find some bad connections etc. But without knowing far more not really able to advise.
Eric
ericmark


Postby marekbzikot » Tue Nov 18, 2008 7:20 am

Hi Eric,
Thank you for your interest. The test method is with a electrical screw driver. My friend who has a multimeter did a continuity test on the cable which all 3 wires became live if only the positive was connected to the live supply wire. This showed that all the test cables had no breaks etc inside them. The problem seems to be upstairs in the garage and I have also noted that the knock out box has current passing through it. The strange thing is that 1) there is less current passing through the earth and neutral then through the live
2) The supply wire from the consumer unit works as it should ie Live is live , neutral and earth have no current passing. 3) if you then connect any cable to this eg via a connector block , you get a current going through L, N and earth wires. It doesn't have to be a block connection, you can get the same effect by simply touching the supply live with the other cable live and suddenly you get all 3 wires, or as many as you have passing a current. 4) this seems to be happening upstairs where the metal knock out boxes are mounted on wood. 5) it makes no difference if the supply is direct from the consumer unit or part of a ring. 6) All the fittings are MK New logic and the consumer unit is brand new Wylex

I got round this problem on one circuit which had a seperate PIR switched outside light by going directly from the the fused socket to an outside light which had an integral PIR as part of the light, hence by passing the need for an intemediary link ( stand alone pir)
The problem now is linking 3 smoke detectors, what I have is a supply going to a patrisse and then of to the 3 detectors by seperate cables . Once again the supply is fine but the moment you join another cable that cables wires all become live. This means that there is a transfer of current across the insulated cables. However I did manage somehow for a short period of time cure the situation on 2 cables by moving the wires around in the patrisse so they weren't touching.
The problem is definately not the wire as I have used brand new wire from different brand new rolls , which were not from the same batch etc
I am looking forward to your diagnosis and appreciate your help
Many thanks again
Marek
marekbzikot
Posts: 4
Joined: Mon Nov 17, 2008 3:57 pm


Postby ericmark » Tue Nov 18, 2008 11:55 am

“all 3 wires became live if only the positive was connected to the live supply wire.” This is normal and is why unused cores should be earthed. It is caused because with AC current you have inductance and capacitance the former is what allows a transformer to work.
Also you can’t measure current with an electrical screw driver it works with so little current it with often give false reading.
L stands for “Line” do remember both Line and Neutral are “Live” wires.
50 meters is quite a long run and it needs terminating correctly at both ends with a gland which will really need to be fixed into a metal box. Although you can put it into plastic with a banjo washer and bolt (brass of course) to connect earth wire the cable is likely to warp the plastic and stop you fitting lids etc. I often fit either a metal surface mount socket box with blank lid or a conduit box under the consumer unit then run the inner cores directly to consumer unit without a joint purely to stop the SWA cable twisting the consumer unit case.
Your maximum feed is about 16A for that length of cable if there are lights in the garage and 25A if only for power because of the volt drop.
You may have problems getting 6mm cable in some single pole RCBO’s and you may need a double pole RCBO which may not fit in some consumer units so you may need to use an enclosure for just the RCBO and it may be better to mount the garage consumer unit in the house with the sides being shorter than normal consumer unit they are less likely to warp and in the garage use a switched fused connection unit with no bigger than 5 amp fuse for lights. Since limited to 16A supply if using lights one would not really want to go below 16A for sockets in garage and no point in having two 16A MCB’s in series. The letters PIR mean Periodic Inspection Report as well as those things alarm engineers use and care is needed using the letters I assume you are using some movement detectors or heat detectors in the garage I can’t really follow what you are doing.
The whole work will need an installation certificate filling in rather than periodic inspection report if that’s what you are referring to although so local authority building controls will accept a periodic inspection report under the Part P regulations. I assume you are aware of Part P if not there is a link in projects section.
Also when you do come to fill in paperwork as required by LABC you will find blank forms on IET web site without ON LINE EDITION printed across them.
Eric
ericmark


Postby marekbzikot » Tue Nov 18, 2008 1:37 pm

Hi Eric,
You are indeed right and now all is well. I am not really bothered about part P as I do not ever intend to sell the farm. Without Part P conformity my successors will be better of as the value of the buildings will be deminished and hence the amout of tax too!!
I am of course very grateful for the time you have given to this query and your extensive knowledge and diagnosis from a distance.
Thank you once again, you are a true star!!
All the best
Marek
marekbzikot
Posts: 4
Joined: Mon Nov 17, 2008 3:57 pm


Postby marekbzikot » Tue Nov 18, 2008 5:13 pm

Hi Eric,
Thanks for the advice , it worked,
All the best
Marek
marekbzikot
Posts: 4
Joined: Mon Nov 17, 2008 3:57 pm


Postby ericmark » Tue Nov 18, 2008 9:50 pm

Glad it worked not sure if a farm comes under Part P it is commercial or at least that would be my excuse!!!
I should have realised with the length of cable. I did some electrical work on a small holding in the Falklands set up battery back-up etc. Seems a long time ago now he had only 5 fields though just 10000 acres he tells me it needs to be 100000 acres to be considered as a farm! he told me 4 acres per sheep so I suppose a little different in UK.
Eric
ericmark


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