Rodent problems

[drywax]

Hi all

Just been to see a customer who's mcb trips out rcd every time switched on first time it happened was in the middle of the night

tested insulation resistance and failed quite dramatically he said he's had a problem with mice so in my mind I'm thinking theres a half chewed cable somewere under the floor does any one know of a simple method of tracing the damaged segmant or is it a case of ripping the floor up until I find it ?

or could it be somthing completley different?

 

[pennywise]

Is it on the power circuit. If so you'll have to break the ring down and test between the sockets until you find the leg with the poor insulation reading.

 

[drywax]

Yeah just thought of that one after I posted and yes its on the downstairs ring but fortunatley I can crawl under the floor I've lost all the christmas excess

 

[kirky]

i would do as the first reply said and break down the ring ! no good crawling around everywhere if you can eliminate by testing !

 

[Spudnik]

You may even find that it isnt rodent damage.

Even with chewed cables the readings would still be quite high.

 

[mechelec]

You may even find that it isnt rodent damage.

Even with chewed cables the readings would still be quite high.

Unless mr mouse has chewed through insulation and is now lying dead accross live and earth!!!

 

[drywax]

located faulty link in the ring seems to be buried in concrete floor, replaced with temp cable to get up and running all works and tests ok IR >300M Ohms need a return visit to replace temp leg all in all time spent in total inc materials I reckon 5hrs +£10 for cable and lifting of certain polished floor boards £135.

Would you say this is a reasonable cost?

 

[Spudnik]

Unless mr mouse has chewed through insulation and is now lying dead accross live and earth!!!

Very true

 

[Jason Smith]

Thats a very reasonable charge. probably a bit on the low side, your whole day was gone, i would of charged for the day and bits, unless it was a regular customer.

 

[RexT]

Hi Drywax,

Do you know the length of the cables and routes? Normally not I guess, so a case of investigation which is expensive for the customer in time and money..

The only thing I can suggest is to breakdown the ring final circuit into its legs and isolate which section by testing which is ok if it is not an intermittant fault.

If it was a break in the conductors I was told of a method for finding breaks in audio cable using a capacitance meter which could pinpoint the break at its distance from the test point, you have to know which conductor is broken and which is good. Then test at each end, divide the capacitance values giving a fraction and then multiply it by the total length of the cable.I havent tried this but it sounds believable.

Hope this helps,
Best wishes,

Rex

 

[drywax]

Yeah may try it some day rext thanks although it may take a week for what you said to sink in

I broke the ring in half IR both legs

found the bad half of the leg

then broke that leg in half did another IR

and continued until I found the bad link

certainly learnt a lot and as electrical cabling is a new field to me goes to prove no matter how much you have in the certificate tray you can't put a price on experience

 

[RexT]

Hi Drywall,

Halving the ring is probably the quickest way but as you say it does still take time.

Hope the householder is going to set some traps for the mice!

Best wishes

Rex

 

[Jason Smith]

Hi Drywax,

Do you know the length of the cables and routes? Normally not I guess, so a case of investigation which is expensive for the customer in time and money..

The only thing I can suggest is to breakdown the ring final circuit into its legs and isolate which section by testing which is ok if it is not an intermittant fault.

If it was a break in the conductors I was told of a method for finding breaks in audio cable using a capacitance meter which could pinpoint the break at its distance from the test point, you have to know which conductor is broken and which is good. Then test at each end, divide the capacitance values giving a fraction and then multiply it by the total length of the cable.I havent tried this but it sounds believable.

Hope this helps,
Best wishes,

Rex

This works, but in real life- not a chance. for instance, I put in 8 meters of 2.5 today to get to a socket a couple 0f meters away from the origin.When dealing with meaured values you need to accurate with cable lengths, its just not possible unless you can see the run, in that case you probably could see the fault.

 

[RexT]

Hi Jason,

Thanks, very good point, have never tried it and remember reading it somewhere or someone mentioning it. It seemed worthwhile mentioning it as it might have helped a bit.
Finding breaks in cables that are concealed and of which the lengths are not known is difficult. I know someone is going to mention that if we know the CSA then we can reasonably know the resistance per metre from page 166 of the On Site Guide however even this can be a bit hit and miss in the real world.
I guess you cant beat the MK1 eyeball!

Best wishes,

Rex

 

[Jason Smith]

Hi Jason,

Thanks, very good point, have never tried it and remember reading it somewhere or someone mentioning it. It seemed worthwhile mentioning it as it might have helped a bit.
Finding breaks in cables that are concealed and of which the lengths are not known is difficult. I know someone is going to mention that if we know the CSA then we can reasonably know the resistance per metre from page 166 of the On Site Guide however even this can be a bit hit and miss in the real world.
I guess you cant beat the MK1 eyeball!

Best wishes,

Rex

your right mate- even if you know where the fault is, you still gotta lift the boards to replace the section of cable------