IR Tests & Fault Finding

[Prime Design]

Hi fellow electricians,


I'll get straight to the point.
How low do the readings for an IR test need to be on 250V or 500V scale before a 30mA RCD or 32A MCB trips when the circuit is live and in operation and where would I find that information? I know it will trip straight away with a dead short but if I get 50M ohms for example on the dead test and then liven the circuit up is it likely to trip after a certain period of time?
Also why does the RCD or MCB not trip straight away? What factors affect the circuit to the point that the RCD or MCB decides it's time to trip if there is not a dead short and there is no faulty appliance?


Finally, can someone confirm that the best way to find the furthest point on a ring in which to split the circuit in half is the socket with the highest Zs.


All contributions are appreciated
Many thanks

 

[Deleted member 72666]

Nah!.... can't be Glenn, just can't be!

 

[telectrix]

Hi fellow electricians,


I'll get straight to the point.
How low do the readings for an IR test need to be on 250V or 500V scale before a 30mA RCD or 32A MCB trips when the circuit is live and in operation and where would I find that information? I know it will trip straight away with a dead short but if I get 50M ohms for example on the dead test and then liven the circuit up is it likely to trip after a certain period of time?
Also why does the RCD or MCB not trip straight away? What factors affect the circuit to the point that the RCD or MCB decides it's time to trip if there is not a dead short and there is no faulty appliance? ohm's law. R=V/I, so R=230/0.03, = 7666.66666667 ohms.
Finally, can someone confirm that the best way to find the furthest point on a ring in which to split the circuit in half is the socket with the highest Zs. not an exact method. better to dead test the resistance on low ohms.


All contributions are appreciated
Many thanks

my comments in red.

 

[Prime Design]

I don't know who Glenn is

 

[nickblake]

zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

 

[ggf]

Keep testing practise makes perfect

 

[Octopus]

hope he got his tin hat on.

 

[telectrix]

i'll ask geordie to lend him his flat cap.

 

[Prime Design]

Thanks Telectrix, took me a while but I finally got it (I think)
So the results need to be as low as 7.6M ohms before the RCD trips. So if I test a circuit and I get 50M ohms why does it trip after a while?

Not sure if Nickblake and Murdoch are taking the mick...

GGF you're only as good as the experience you've had lol I'll keep testing...

 

[telectrix]

not 7.6 meg. 7.667 K. RCD will never trip at 50 Meg. BS 7671 states thatr a reading of 1Meg. is acceptable.

 

[Plonker 3]

As been said before I=V/R

Then use the tables in Appendix 3 to work out tripping times of the OCPD

 

[overskilled]

no 7666 OHMS not MOhms...
intermittent fault, damp ingress into a light fitting, chafed cable etc etc (had a weird one once, only tripped at random times, checked a light fitting which was alright...looked above...with the torch...copper glinting back in the darkness...socket cable had been damaged through chafing, and everytime someone trod on the floor above the cable it was enough to move the cable and cause the rcd to ping out...)

 

[Prime Design]

See I was always too embarrassed to ask how many digits in 1 meg, I kinda feel stupid now but I better get it straight before I move on, 1st year stuff I know.
1000 ohms is 1 Kohm? 1,000,000 ohms is 1 Mohm?
So when my meter says >999 M it's greater than 999 million ohms?

Thanks overskilled

I have a megger 1730, I done a fault find today and IR said 0.51 on the ring. After an hour and a half of investigation I found a cupboard behind the f/f with a neon switch in the on position. Turned it off and it took my readings up to 160. So I put everything back together everything still unplugged and switched off and results dropped to 50. So this is why I'm asking these questions. I have to go back and rectify the fault but just wanted confirmation the RCD is not going to trip in the middle of the night.

 

[telectrix]

anything over 2 Meg. you can assume to be OK. and them neons are a pain if hidden. anothe fav. for false readings is an aerial amp. in the attic.

 

[Prime Design]

Cheers Telectrix