Socket Zs High

[AdieB]

Sorry tried to edit my last post but it did not appear.

The socket is part of the ring circuit. The cabling is exactly the same on all sockets. It is a TNC-s system, I pushed the plug of the tester in and out numerious times so hopefully no tarnished connections. I also took reading from rear of socket. The DB is a MCG fuseboard with a 2P6330 RCD and 2 x 60898 mcb’s 1x 20Amp 1 x 6Amp. What is the table or reg that says about rcd controlled socket can be 1667 ohms.

 

[Geoffsd]

Am I missing something?

Shouldn't the reading be 0.35 (or even less with parallel paths) at all the sockets?


1666 ensures the RCD will disconnect before voltage rises above 50V.
It should not be used to ignore maximum Zs values - unless unavoidable (e.g. TT)

 

[telectrix]

table 41.5, p50 in the red book. not sure where in bgb.

 

[spark 68]

pushed the plug of the tester in and out numerious times so hopefully no tarnished connections

I meant where the cables terminate in the back of the SO too, as your end to end readings were ok.

What is the table or reg that says about rcd controlled socket can be 1667 ohms.

Table 41.5 (BGB) p57.

It actually says anything above 200 may be unstable wrt Earth rods.

Edit: Tel beat me to it, again! lol

 

[johnboy6083]

the 1667Ω figure is for earth faults, and not for live-n faults. I would carry out a Zp-n, which means that you test between live and neutral, not live and earth. If your using a fluke, its just a case of swapping the leads over, but if its a megger, then youll have to use the probes. If this comes up ok, then youve identified that it is indeed a problem with the earth path, and not the live. If it comes up bad, then it means that you have problems wiuth either your live or neutral.

 

[Jonny66]

Am I missing something?

Shouldn't the reading be 0.35 (or even less with parallel paths) at all the sockets?

Totally agree, should be getting a Zs of around 0.35ohms at each socket outlet. Sounds like a loose connection at one of the sockets.....

 

[Moses]

As its a socket should be on a RCD anyway, in which case Zs up to 1667ohms.

Not Correct!

RCD is backup / supplementary protection to the main protection.

Will the RCd trip the circuit? when there is a shortcircuit? and the cable burns the house down?
because the CB did not trip, because the Zs was too high?:laugh3:

 

[Moses]

Totally agree, should be getting a Zs of around 0.35ohms at each socket outlet. Sounds like a loose connection at one of the sockets.....

Zs = (r1+r2)/4

 

[Jonny66]

Zs = (r1+r2)/4

Does it ??????? Zs = Ze + (R1 + R2)..........in this case Zs = 0.28 + 0.07 = 0.35ohms

 

[PC Electrics]

Zs = (r1+r2)/4

My turn to correct you, Moses:

R1+R2 = (r1+r2)/4
and
Zs = Ze + (R1+R2)

so
Zs=((r1+r2)/4)+Ze

 

[Moses]

Does it ??????? Zs = Ze + (R1 + R2)..........in this case Zs = 0.28 + 0.07 = 0.35ohms

My mistake!

R1+R2 = (r1+r2)/4

 

[PC Electrics]

This log cabin, does it have it's own metered supply? Or, is it fed by a distribution circuit from another building?

If the latter, then what is this Ze you quote? Ze at the supply or at the cabin board (ie Zdb)?

Looks to me like we're missing some info.

Anyway up, those Zs figures (assuming that's what they are, you don't state explicitly) are somewhat astray. Poor or varying Zs at sockets can be due to bad connections at the pins or dirt/corrosion on the switch contacts. Have you tried flicking the switches a few times to 'clear' the contacts?

 

[yellowvanman]

Not Correct!

RCD is backup / supplementary protection to the main protection.

Will the RCd trip the circuit? when there is a shortcircuit? and the cable burns the house down?
because the CB did not trip, because the Zs was too high?:laugh3:

I'm not disagreeing with the need to try and find out why the Zs is high but:

411.4.4 - An RCD can be used for fault protection with an overcurrent protective device also in place.

When does anybody measure the Z between neutral and line to ensure that an MCB will trip? The only measurement you make is Isc at the origin!

 

[GLENNSPARK]

Not Correct!

RCD is backup / supplementary protection to the main protection.

Will the RCd trip the circuit? when there is a shortcircuit? and the cable burns the house down?
because the CB did not trip, because the Zs was too high?:laugh3:

your not measuring line voltage or short circuit with a Zs Moses.....

 

[GLENNSPARK]

and these `magic` RCDs should not be seen as a `cure all` for high loop......this does need investigating....