Earth loop impedance on TT?

[nogood]

Hello all,
Plugged in Martindale EZ150 and all wiring OK except loop test of between 10 and 100 Ohms - Is this acceptable on TT? Also, when isolating lighting circuit on CU and cutting the isolated wire to put in new light it tripped RCD - Would this be due to shorting on an earth leaky circuit?
Thanks to all you helpful people!!!

 

[Dellsmp]

Tripping rcd is probably down to shorting neutral and earth when you cut the cable, mcb isolation only disconnects live. so everything working as it should, ok

 

[Adam_151]

Thats fine, however the unstability of it bothers me, yes it will flucuate a bit, but I'm supprised at that.

RCD is sized according to touch voltage, multiply the electrode resistance by the operating current and you need to get a value of below 50v (below 25v on farms, etc). a value of over 200 ohms is not recommended because it is quite likely to be unstable

 

[nogood]

Dellsmp - Simply the BEST!!!!!
You, my cyber genius, and all other good people!!!!!
If I turned off all 32 MCB's (all rings) (no higher rated ones on board) and the RCD tripped could this be due earth leakage somewhere? As said on previous post loop test of between 10 and 100 Ohms (can't afford £100's on megga,etc). Earth rod in hard, damp clay - don't know if this is a good thing or not. Do I need another linked earth rod maybe to stop tripping on all 32 MCB's turned off?
Thanks.

 

[nogood]

Adam151 -
posts crossed...You are obviously well hung!!!! Ta and thanks!

 

[Adam_151]

How did you test the electrode?

EDIT: NM, I'll learn to read ppropelry one day.... lol

 

[nogood]

Lots and lots and lots of laughs - Don't know how to shorten that!! Why would the RCD trip when 32 amp MCB's turned off though?
YOU DA MAN

 

[BSBS]

The RCD is tripping on the circuit when the MCB is turned of becuase the neutral is still connected for that circuit so when you cut the cable you are creating a Neutral Earth fault which is tripping the RCD.. the ony way to stop it is to disconnect the corrosponding neutral form the board for the circuit you are working on.. or try to cut the cable without cutting the neutral and earth at the same time

 

[securespark]

With a 30mA RCD, you can have a maximum of 1666 Ohms EFLI.

Thats fine, however the unstability of it bothers me, yes it will flucuate a bit, but I'm supprised at that

Think you misunderstand...the reading is not fluctuating, the tester he has lights up a neon giving a range of 10-100Ohms.

 

[nogood]

Hello all, just to clarify a bit as I din't think I've explained myself properly-
The RCD is rated at 63A and it trips when just turning off 32A MCB (the only ring on CU at the moment, no shower, cooker, etc yet). I don't need to cut any wires for it to trip the RCD it just trips of its own free will LOL! Bit of a pain when trying to isolate 1x ring and ending up with no lights, resetted clocks, etc.
Was wondering whether it was due to earth leakage somewhere?

 

[Spark123]

With a 30mA RCD, you can have a maximum of 1666 Ohms EFLI.

I thought the same as adam, the reading should not exceed 200ohms for stability.

 

[securespark]

Hello all, just to clarify a bit as I din't think I've explained myself properly-
The RCD is rated at 63A and it trips when just turning off 32A MCB (the only ring on CU at the moment, no shower, cooker, etc yet). I don't need to cut any wires for it to trip the RCD it just trips of its own free will LOL! Bit of a pain when trying to isolate 1x ring and ending up with no lights, resetted clocks, etc.
Was wondering whether it was due to earth leakage somewhere?

The last time I had this problem was on a board change I did. The split side RCD tripped when I switched off the main isolator.

I found an appliance plug wired with E/N reversal. Worth a look round. Certainly start by disconning all appliances from the circuit, as well as switching off all DP switches. Don't just switch off socket switches, unplug!!

Then see if the problem goes away...

 

[securespark]

With a 30mA RCD, you can have a maximum of 1666 Ohms EFLI.

I thought the same as adam, the reading should not exceed 200ohms for stability.

Isn't that on TN systems? Look in OSG...

 

[Spark123]

Nope, definately TT systems. Think its in GN5.