Zs reading help?

[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.

 

[platty1005]

Ringer all installation work should comply with BS7671 and ALL work should be inspected tested and certified with at least a Minor Works Certificate. You have got to make sure the installation is safe and the MWC is your proof if there are problems later

 

[Deleted member 9648]

Ringer all installation work should comply with BS7671 and ALL work should be inspected tested and certified with at least a Minor Works Certificate. You have got to make sure the installation is safe and the MWC is your proof if there are problems later

Yes...but like for like replacement is not classed as installation,it is classed as maintenance,IMO there is no requirement to issue a M/W for a like for like replacement.
Of course it may be worthwhile doing so...but not essential.

 

[platty1005]

Sorry wire puller its obviously a difference of opinion I would say you are INSTALLING a new shower not maintaining an old one. You are disconnecting and reconnecting a circuit cable so how can you be sure it still complies after your work. Better still what if the circuit was faulty in the first place and you just replace the shower unit with the fault still there. Read page 336 of the 17th. This certificate may be used for the replacement of equipment such as accessories or luminaires, but not for the replacement of DB's or similar items. Appropriate inspection and testing, should ALWAYS be carried out irrespective of the extent of the work undertaken. So if you bother to inspect and test you should always record the findings in this case MWC

 

[stevemb60]

Hi
When you say the RCD can introduce a resistance, when the circuit has been proved ok with a r1+r2 calculation is it faulty (needs replacing) or just causes incorrect readings?

 

[telectrix]

as the max. Zs with a 30mA RCD is 1667 ohms, you can leave the RCD alone. but if you want to get the Zs down to a value where the ocpd will break within the specified time in the event of a fault of negligible impedance, then you need to replace it.

 

[stevemb60]

Thx Telectrix,
So if I've understood you correctly with an RCD you can have above spec Zs values which meet regulations, but you wont reach times to trip over current faults which may cause fire etc.. So I guess the Regs state its ok but for safety you do your very best to get the correct OC Zs.

 

[abbi normal]

ran this problem past the NICEIC man ..... he said it was because of the no trip loop tester .... and as long as the R1+R2 and Ze are ok you can use the calculated method or if the Zs results are beyond the mcb max value use the magic 1667 as the result .. I hadnt had the problem with my old robin triple set up ...and didnt take the no trip thing into account when getting high readings .... after going through the wiring and connections on an installation a few time ...checking and re checking .... as a last resort I checked the Ze before the RCD then after the RCD there was the difference ..... I have found that different makes cause different results ..... for example ...MK ... there is a difference but the results are within the max for the mcb ...Hagar .... same as MK .... CED and contactum ... difference caused me to check all the connections on my installation in the first place .... hope that helps ....

 

[Roger]

Ok I must confess never encountered high Zs reading this large but the 1667 ohms is for TT systems and reading this post i thought it was a tncs earthing system so it should be in line with table 41.3 to achieve disconnection time.
Did you achieve the required disconnection time ? what did you calculate the Zs to be with your R1 and R2 readings.

 

[spark 68]

Hi Roger,

but the 1667 ohms is for TT systems and reading this post i thought it was a tncs earthing system so it should be in line with table 41.3 to achieve disconnection time.

If your Zs is on the high side, due to say a long cable run or other reasons, fitting an RCD will mitigate this, as then you are allowed upto 1667 ohms, regardless of the earthing system in use.
But having said that, I would always try to keep my Zs values as close as possible to the max given in the BRB/BGB/OSG.
I have known other sparks to use an RCD as a 'Magic bullet' instead of investigating why a Zs reading is high, or at least calculating it to see if it is within the expected value (ie. a high R1+R2).

WRT

Hi
When you say the RCD can introduce a resistance, when the circuit has been proved ok with a r1+r2 calculation is it faulty (needs replacing) or just causes incorrect readings?

The RCD in question was adding the resistance (false reading) to the no trip reading, but the calculated ie, measured Ze and Measured R1+R2, added together were fine and within bounds, just the RCD was giving spurious readings, I changed it because the one on the otherside of the board was not giving the misleading results, better safe than sorry and all that.

 

[Guest123]

Ok I must confess never encountered high Zs reading this large but the 1667 ohms is for TT systems

The 1667 ohms value applies to the actual 30mA residual current device REGARDLESS of what system it's installed in.

The figure is derived from the calculation concerning the time taken to clear an earth fault before the safe touch voltage of 50V is exceeded.

50/0.03 = 1666.67.

That said, in any TN system, or TT for that matter, the earth fault loop should not be anywhere near that figure as that really is worst case value for that particular device.

The max values for other RCD's can be calculated in the same way,

50/0.1 = 500ohms (100mA)
50/0.3 = 167ohms (300mA)

etc.