Testing with a Megger

alw94 · Aug 22, 2009

Hi,

a question now on testing with a Megger. I've been practising on a dead circuits on a house renovation. But to be honest I'm struggling to use and understand the Megger and results and relate them back to what I learned on the course. I'm probably being dumb about it all but some help would be welcome.

1) Continuity of protective conductors - I'm sure the Megger is very easy to use and it does seem easy but I don't understand that way the instructions say how to do it. Can someone give a idiots guide to testing for this part?

2) Continuity of Ring Final Circuits - end to end tests seem to be consistant with 1 circuit having L - .27, N - 0.28 & CPC - 0.45 another having 0.26, 0.27 & 0.43.

Tests at sockets seem ok with each circuit with results consistant, around 0.22-0.29, and within 0.05 of each other.

3) Insulation Resistance - testing at C/U I'm getting >299m ohms while testing on 250 although some seem to creep up slower to reach it than others, problem? Also on boiler and light circuits I'm getting a reading of around 11.2 why? It must be something stupid I'm doing.

I have got electricians to go to for help but I'm trying to sort this out myself before using them as a last resort

Many thanks in advance.

Sintra · Aug 22, 2009

What model of meggar are you using?

As for your IR tests have you removed all lamps etc if you are testing between L & N. You should be testing at 500V not 250V

ezzzekiel · Aug 23, 2009

1) Continuity of protective conductors - put temp link between live and earth set megger to continuity (orange scale on mfts) test between live and earth at outlets with red/green probes checks ncontinuity R1+R2 and polarity

2) Continuity of Ring Final Circuits - results are fine

3) Insulation Resistance - test at 500v on lv systems readings need to be greater than 2M although in reality we expect much higher, low or 0 readings are usually caused by things left in circuit ensure everything is of, neons, lamps, hidden stuff behind units etc etc..

alw94 · Aug 23, 2009

Many thanks for both posts. I'm along the rights lines and things are a little clearer. I'll do some more testing.

The only part I'm still on sure on is the contunuity tests. Not so much about putting in the link or testing but how I use the Megger, basically which setting on the orange scale. I dont have the megger instructions in front of me but off the top of my head, if I follow the instructions, set tester to continuity, check at switch, the buzzer goes off, my question here is at what ohm rating should I be testiing at? or am I testing on the wrong part of the orange scale? I have managed to get results but just not confident what they are. I'll try again today and see what I get

saveloy · Aug 23, 2009

Megger MFT 1552/1553 on the continuity scale (orange) you need to select the ohms setting (next one down from Volts to take a continuity reading. The next setting (buzzer) just proves continuity, but won't give you an ohmic value, other than >20 ohms.

Make sure that you also null yuor leads out first otherwise your ohms reading will be false. To null out the leads you need to put the 2 probes together an press the yellow button on the top at the same time, whilst on the continuity setting. An O symbol with an arrow over the top will display on your screen to indicate the leads are nulled. The value displayed should also read 0.00 when your probes are touching.

alw94 · Aug 23, 2009

Ah yes, now it is fitting together better. Got the results I wanted for the IR - in the end it was the fused unit for the boiler that I hadn't disconnected. All circuits >299.

Tested a light circuit just as you stated earlier, Saveloy (thanks just the idiot guide I needed), as I thought that must be the way. Didn't quite get the results I was hoping for but I'll try again tomorrow. At least I know now the right way to do it. Losing too much hair what with all the head-scratching!