Zs reading help?

[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.

 

[Roger]

Ok I understand how it been calculated as the maths could not be easier.Blimey i've spent time getting worried when my Zs is 2 ohms higher than the required 1.44 on a 32amp RCD protected ring.
The table 41.5 is listed under the TT system and that is what I have always worked to.
Reading the small print Note
The circuit loop impedances given in the table should not be exceeded when the conductors are at there normal operating temperature.
Like the previous post RCD/RCBo's are the 'magic bullet' or get out of jail card.

Never stop learning,now to play with my new Megger 1720 on the house.

 

[Blowcat]

Hi guys,
I had a job this morning to change a faulty shower unit in a flat with a direct replacment. when I had fitted the replacment unit I started the minor works cert. Tested the insulation resistance on the 10mm T+E cable all fine. But when I did then Zs I had a reading of 595 ohm? Having messed about checking the conections on the probes on the meter and what not I just thought I would test another circuit to compare,and sure enough same round about reading. Did a Ze on the board to make sure the incoming earthing was up to scratch and got a reading of 0.12 ohm (TNS Supply). What could cause this kind of reading on the Zs??:thinking:

Assuming all connections are good and your R1+R2 is roughly what you expected, I would do a Zs reading at your incoming supply using the tester on the RCD range, to see if your incoming supply is getting a good Zs on the RCD Zs range.