Using a 'Megger'

[camillitech]

Bit of a problem on the croft chaps,

the old TraceSW4548e keeps shutting down and a red LED comes on saying 'over current', which means one of two things, overload or short circuit. Well it's certainly not overloaded so it must be shorting somewhere, not that I'm any kind of expert but the same thing happened five years ago and it was the devils own job to find the problem. Isolating the various circuits on six sheds, the house, etc,etc, not helped by the fact that it could go five minutes or fifteen hours between tripping out.

Quite by chance I found it by being in the right place at the right time and heard the crack as the lights went out, turned out to be some T&E gnawed by a mouse 

So after five years without a power cut it happened again just after midnight on Tuesday so me suspects another mouse in my warren of sheds with their spaghetti wiring  Of course I'm now at work until Tuesday night so can do little more than check the obvious (which I've done)

Speaking to one of our electricians he suggested a Megger and posted one to me, It's an AVO BM80/2 from RS. Of course there's no instructions, a search on the net had me baffled, there's no phone signal at our house and I've a memory like a hen.

So in words of one syllable how do I go about testing for leaks, shorts etc, I've isolated most circuits and it would appear to be between the generator shed and house and not in any of the other outbuildings. More likely than not between there and the fuse box in the house, so before I start digging up the cable I'd like to be sure 

Many thanks in advance, Paul

[Richard Owen]

My competent person, who happens to own and use a Megger, is out boyfriending all weekend.

She's back Tuesday and I'm sure she'd be delighted to help.

I've PM'd you here mobile number and email address.

[daftlad]

Ok A megger measures resistance, there will be 2 main settings megga ohms and ohms, megga ohms is what you need.
A megger measures insulation resistance, to accurately test the resistance it uses a high voltage to test damaged insulation. There are sometimes different voltage settings, I would recommend 250 volts to start, so you don't blow up appliances that you have accidentally left connected.

Because it is a short or an overload and you don't have an RCD tripping then the fault must be between live and neutral or live and earth.
So with the cable disconnected or switched off with a 2 pole switch connect the probes/ clips to the 2 wires that you want to test between and press the button, The resistance should be more than 2 Mohms but realistically it could register as infinity ie greater than the maximum that the meter can measure.
If you can only test at 500 volts then be careful not to blow up expensive equipment  

peas

[firepete]

normally you disconnect the wiring ends . switch megger to mohm scale clip one wire to each terminal in turn and the other to earth wire. and push button. you should have a infinity reading if it is good.1mega ohm about minimum .you can test between wires too. no touch the terminals or your get a shock.


beat me to it

[camillitech]

Many many thanks chaps, seems quite straight forward, will keep you posted with developments, a full 12 hours and thirty minutes since the last power cut  Hopefully I'll get a chance to have a look on Sunday. I did do a little reading up before I asked you guys and it all sounded so complicated but basically what I do is disconnect the wire I'm testing at both ends from anything else and fire some high voltage doon the wire  Just one wee question, what makes the voltage  I see on the old ones that you just crank them up but this is digital with a button, what sort of battery does it use 

Cheers, Paul

[psutherland71]

Hi

An insulation resistance teater just works off normal batteries (mine has 6 x AA), I'm not sure how they step up the voltage but I assume there will be an inverter circuit, transformer and rectifier. Although the voltage is high there is very little current so although you may get a fright if you touch the probes it won't hurt you, car spark plugs give a worse tingle.
You should set the tester to the 500v setting, disconnect cable at both ends, test live to earth, neutral to earth and live to neutral, all readings should be at least 1 Mohm but if the cable is good should really be hundreds of Mohms.
Also on long cable runs the cable can remain charged for a short time after the tester is removed as the longer the cable the more capacitance it has and chrges up just like a capacitor.

Peter

[Justme]

Also on long cable runs the cable can remain charged for a short time after the tester is removed as the longer the cable the more capacitance it has and chrges up just like a capacitor.

Spoil sport 

[SimonHobson]

It's quicker to start with a whole installation/section of it.

Make sure you disconnect any electronics - unplug all appliances, switch off anything that's hard wired (or disconnect it). It's best to have all lights switched on so you can test all the wiring to them - if a light is switched off then some of the wiring to it isn't live.

Then turn off the main switch on the fusebox/consumer unit. Link live and neutral together, and test with the megger between live+neutral and earth. You need to use 500V because 250V is less than the peak voltage of the mains (which is nominally about 1.4 * 240V, or about 330 to 340V) - so testing at 250V wouldn't stress things as much as the mains. If you get a good reading (over 2M is considered adequate) then the whole system supplied by that board is OK and there's no point testing bits of it individually.

If you get a low reading, then you need to start dividing until you find the problem. One circuit at a time, drop the ends out of the circuit breaker/fuse and neutral bar (or neutral terminal of the RCBO is using those), put one tester clip on the live(s) and neutral(s) together and the other on the earth bar and test. If it's bad then investigate, otherwise move to the next circuit.

If you do have RCBOs (ie combined overcurrent and residual current protection) then you need to go circuit by circuit as the manufacturers usually instruct not to megger these - hence you need to drop the circuit tails out and test separately.

IMPORTANT SAFETY WARNING
Hopefully I don't need to say this, but just in case ...
Sticking your fingers in to a live fuseboard/consumer unit is dangerous. Switch off at the main switch, and ideally lock it off while you are working - and check that the live feed is indeed gone from the main busbar. The feed terminals to the main switch should shrouded so you can't touch the live feed - but it's worth checking that this is the case.

[camillitech]

Cheers for all the help chaps, sorted it yesterday,



and here's the culprit, a bodged joint in the undergrowth, says a lot for 'Densotape' as it's before my time and I've been here over 22 years http://lifeattheendoftheroad.wordpress.com/2012/02/01/six-by-two-anyone/  I removed the connections from the the fuse board in the house first as I'd a feeling the problem was in the cable between the generator shed and the house. It seemed to be much worse when wet and this was the only external cable that I'd not isolated previously.

Had really good fun with the 'Megger' and learned much  Good to know that I can get my hands on it at short notice if I should ever need it again 

Thanks, Paul

[clivejo]

Nice one, I would enjoy playing with that!  How much are they?

[SimonHobson]

Anything from "a few quid" to "a good few hundred quid"

Dad has an old Megger BM-10 he picked up when somewhere he worked with was having a clearout. Paid virtually nothing for it. It's an old analogue meter that only does IR and continuity.

At the other extreme, you can now buy multifunction testers that do this - plus measure voltage, test MCBs and RCDs, calculate prospective fault and short circuit currents, etc, etc, etc. New they run to at least half a grand, just occasionally you may pick one up for a good price - but no, mine isn't for sale.

If you are on good terms with a spark, it may be worth asking him to keep an eye out. Over the years most electricians have had to keep updating their test gear to keep up with changing regs - many will have kept their old gear "as a backup", and occasionally it will get disposed of. If your friendly spark lets it be known he's looking for one, then it may get offered his way.