earth loop impedance

[oddball01]

hi
can anyone tell me,if i get a high reading on an earth loop impedance test on a socket outlet does this simply mean that the there is poor earth continuity ? and what is a likely cause. [/b]

 

[ColJack]

could be any number of reasons..

cable size too small, cable run too long, lose or burnt connections on live or earth, broken leg on a ring etc...
could also be a socket fed of a fused spur from a ring with the fuse raising the live resistance...

disconnect from board and check circuit continuity and compare values..

earth value should be 3/5ths ( ration of 1.5 to 2.5 ) the value of the live and neutral.

 

[Chivers7]

could also be a faulty socket.

Are you able to / capable of measuring R1 + R2 of the circuit?

Have you measured other Zs off other circuits and if so what is the difference.

Have you measured the Ze?

 

[davelx]

could be any number of reasons..

cable size too small, cable run too long, lose or burnt connections on live or earth, broken leg on a ring etc...
could also be a socket fed of a fused spur from a ring with the fuse raising the live resistance...

disconnect from board and check circuit continuity and compare values..

earth value should be 3/5ths ( ration of 1.5 to 2.5 ) the value of the live and neutral.

Other way up I think - 2.5 to 1.5 since the earth conductor is smaller - so its resistance will be higher - 1.67xLive for 2.5T&E

 

[securespark]

could also be a faulty socket.

Yes. Measure the Zs from the terminals behind the socket front.

I had a maintenance call to a house with high Zs on the ring final. The engineer had tested one socket, got a low reading, and tested 3 more, all low. He came to the conclusion that the circuit was down.

I get there and discover the Zs is fine from the socket terminals. All the sockets in that room (60's Volex) were sloppy, causing very poor readings.

 

[ColJack]

Other way up I think - 2.5 to 1.5 since the earth conductor is smaller - so its resistance will be higher - 1.67xLive for 2.5T&E

yeah that's what i meant.. hadn't had my coffee...

I get there and discover the Zs is fine from the socket terminals. All the sockets in that room (60's Volex) were sloppy, causing very poor readings.

isn't that the point though..

if it's a high reading at the socket front, and the fault is on the appliance then the Zs will still be high back to the breaker causing longer disconnect times?

although theoretically it should take the fuse in the plug first..

if it's good on the terminals but bad on the socket front I would recommend changing the socket outlets to new ones..

 

[oddball01]

thanks very much guys

 

[securespark]

I get there and discover the Zs is fine from the socket terminals. All the sockets in that room (60's Volex) were sloppy, causing very poor readings.

isn't that the point though..

if it's a high reading at the socket front, and the fault is on the appliance then the Zs will still be high back to the breaker causing longer disconnect times?

The Zs in my case wasn't high, it was non-existent.

although theoretically it should take the fuse in the plug first..

Should it?

if it's good on the terminals but bad on the socket front I would recommend changing the socket outlets to new ones..

That's what I did...

 

[ColJack]

yes it should..
discrimination..

a fault should take out the first over current device it comes to in a circuit.. otherwise whats's the point having it..?

you don't put 13 amp fuses on a 6 amp mcb do you?

 

[davy_owen_88]

A BS1362 fuse will rarely discriminate against an MCB during an earth fault.

 

[Taylortwocities]

Mates he's got high Zs. If its that high the fuse isn't going to blow in the event of an earth fault.

TTC

 

[mst]

answer is....... could be anything,if its a minor works job add an rcd which should be there anyway.

 

[securespark]

Mates he's got high Zs. If its that high the fuse isn't going to blow in the event of an earth fault.

That was what I was getting at.

If the loop's that high, nothing's gonna blow, the 1362, the breaker, nowt.

 

[ColJack]

that's what I was getting at too

if the Zs at the terminals is fine, but at the socket front is high because of a worn socket or dodgy switch then the fuse / breaker aint gonna blow fast enough..

 

[securespark]

yes it should..
discrimination..

Is what you said...

Do I sense a little back pedalling?