Zs reading help?

[telectrix]

yeah. i'd make sure of the connections. as its affecting all circuits, any fault has to be at or close to the CU, then replace the RCD and see what effect that had on the Zs.

 

[spark 68]

I suppose a quick way of determining if it was the RCD at fault, would be to open the RCD/RCBO then join the CPC and L at the far end and then loop test across the open RCD/RCBO.

 

[Deleted member 9648]

i posted a similar scenario. Ze of 0.2 ish and a Zs of 350 every circuit. it was the crappy RCD.

I dont get this,and have never experienced it myself.....Surely there is a direct CPC path to the MET,so there can be no excessive resistance there,and if you had a resistance of hundreds of ohms through the RCD contacts any loads connected to it would not work??
Perhaps the OP's shower was not faulty but it was the RCD's high resistance contacts preventing it from working properly??

 

[telectrix]

I dont get this,and have never experienced it myself.....Surely there is a direct CPC path to the MET,so there can be no excessive resistance there,and if you had a resistance of hundreds of ohms through the RCD contacts any loads connected to it would not work??
Perhaps the OP's shower was not faulty but it was the RCD's high resistance contacts preventing it from working properly??

the one i came across had Zs of 351 on all circuits. measuring on the outgoing side of the RCD gave a reading of 349.odd. but everything had been working fine for years. Ze was 0.2

 

[liddley]

I have never experienced this high readings before now. I work for a local authority and we use the same setup in every property. Hager disboard with hager Bs 61009 RCBO`s .I just found it strange how all the circuits gave me high readings. My metral tester has never failed testing circuits fed by RCBO`s before and I thouht It would be highly unlikely all 8 of the RCBO`s would be faulty.
I questioned the max Ze in my last post just to make sure I was right in thinking it is 0.8 ohm for a TNS system, as most of the dwellings I work in have been upgraded to a TNCS supply.
The shower unit did not have an electrical fault on the unit it was damaged by frost in the cold weather last year an never got replaced.
All connections are tight within the consumer unit as this is the first thing I checked when I got the high readings. Was not sure on how to combat this w ith my limited testing experience.

 

[JUD]

I dont get this,and have never experienced it myself.....Surely there is a direct CPC path to the MET,so there can be no excessive resistance there,and if you had a resistance of hundreds of ohms through the RCD contacts any loads connected to it would not work??
Perhaps the OP's shower was not faulty but it was the RCD's high resistance contacts preventing it from working properly??

Reading through the posts, I was thinking the very same thing.

The volt drop would be enormous.

 

[telectrix]

strange though, it isn't. it's not like the RCD had a huge resistor inside it, it's just the internal magics of the RCD that throw the Zs readings to pot.

 

[spark 68]

I had one RCD that 'added' about 70 ohms to all of that side of the split board, I even swapped it over with the other one on the otherside, and the duff reading swapped with it.

As you say the volt drop should be very large, but it wasn't, I just replaced it, I wonder if it was noisy electrically or something ???, having said that my loop tester has the facility to show excessive noise and didn't, I also searched to see if I had left something inadvertantly powered up, so pass on that one, better safe than sorry

 

[tony mc]

I have encountered the same problem a few times when doing the live ZS.

Is it not something to do with the small amount current put in on a No trip loop that is causing the problem and it only happens on the cheaper range of RCD's.

Every time I have bypassed the RCD on the Hi loop the test comes back as expected from the dead tests and ZE readings!

I am sure someone on the forum with in depth knowledge on the subject will be able to explain!

 

[telectrix]

a similar thing happens with intruder alarms. years ago, when the panels were operated with a key, you could put your multimeter ( on ohms) across the zone pair/s with the zones still connected in the panel, and get a reading of the zone loop, usually under 10-15ohms. with the modern panels, you read silly values in the kohms region, even though the zone loop is under 15ohms. dis the pair from the panel and you get sensible readings.

 

[platty1005]

If more than 1 circuit is affected it sounds like a problem on the earthing conductor from method of earthing TNS to MET to earthing bar in CU where did you measure the Ze ? and did you do a continuity from the earth cable where you measured Ze to the earthing bar in CU where the CPC's are connected

 

[ringer]

Putting the discrepancies on text readings aside for the moment, if you were replacing an appliance like-for-like then why are you testing and issuing a MWC? You have not altered the circuit, you are only replacing the appliance.

 

[Octopus]

Putting the discrepancies on text readings aside for the moment, if you were replacing an appliance like-for-like then why are you testing and issuing a MWC? You have not altered the circuit, you are only replacing the appliance.

Even on a like for like replacement, surely you test the ELFI don't you??

 

[ringer]

Yes Murdoch, I would satisfy myself that the supply and characteristics are adequate, but because I have not altered the electrical installation I see no need for a certificate.

 

[spark 68]

If more than 1 circuit is affected it sounds like a problem on the earthing conductor from method of earthing TNS to MET to earthing bar in CU where did you measure the Ze ? and did you do a continuity from the earth cable where you measured Ze to the earthing bar in CU where the CPC's are connected​

If I had been testing for Ze (which may not have been neccessary in this case), I was always taught to disconnect at the earthing bar of the CU (where possible), and test here, to take into account this possibilty, or to at least verify the connection between the MET and the CU as you would do as part of your normal testing routine, ie. verifying protective conductors.

Doing this would the eliminate that possibility.