Couple of testing questions

[studentspark]

Zs on a socket ring circuit (32 A type B MCB TNCS) is 1.5 ohms, which is above the limit in the OSG.

The circuit in question is protected by a 30mA RCD.

I plan to test all the S.O. for loose connections, but does the RCD protection now diminish the importance of the Zs test results.

Any guidance would be appreciated.


Mains voltage fan above shower 220mm from tray, not RCD protected.

If the fan is IPX4 or greater, and was installed on an RCD, would that comply?

 

[ricicle]

On a TNCS system I would want to know if the high Zs was down to circuit resistance (poor design) or a bad connection. IMO it is poor practice to rely on an RCD as a solution to a problem that could have been designed out !

 

[dingbat]

Mains voltage fan above shower 220mm from tray, not RCD protected.

If the fan is IPX4 or greater, and was installed on an RCD, would that comply?

Are you saying it is less than a foot above the shower tray, or did you miss off a zero? Or is that a horizontal distance from the tray, making it Zone 2?

Read Reg 701.411.3.3

 

[BSBS]

Before taking and sockets off you would be best of doing Continuity tests on the conductors and R1+R2.

 

[riveralt]

Before taking and sockets off you would be best of doing Continuity tests on the conductors and R1+R2.

Agree absolutely - what was your r1, r2, rn by the way?

I assume that the cable size is 2.5mm for the ring final circuit.

And what was the external loop impedance value?

Or have you just done a Zs to check on the circuit?

 

[ericmark]

The question of ELI with use of RCD has been talked about many times. True the earth loop is not important as long as under 200 ohms but the volt drop requirements are very similar to earth loop requirements so with a 32A supply at 6.9v = 0.22 ohms line / neutral impedance. Also with a line / neutral fault one would hope that the MCB would trip on magnetic part of trip so with for example a B type MCB then 160A (5 x 32) must be able to flow so 230/160 = 1.4375 ohms.

As already talked about one worries about bad connections when the ELI is high. The same applies to line/neutral impedance and if the measured impedance exceeded 1.4 my worry would be why.

Some meters auto measure the PSC current between Line/Neutral others require one to make DIY leads or use probes to check between line and neutral. You have to know your own meter.

If Line/Neutral reading under 160A or 1.44 ohms then I would consider down rating MCB but not until I had looked at likely cable run and likely hood of too much cable being used. Of course take incoming reading if the PSC current is below 660A or 0.35 ohms if impedance measured then I would inform DNO and write DNO informed of low PSC on installation cert and suggest it is re-tested once DNO had corrected fault.

A 60A supply with +10% and -6% ie 36.8v = 0.61 ohms which is max DNO can get away with on TT supply. At 100A this drops to 0.368 ohms so when they say 0.35 ohms for TN-C-S not far away form max volt drop reading anyway.

 

[studentspark]

Sorry 220cm. re fan above shower


Looking at my test sheet now (was going from memory in first post) for circuit that caused concern

Zs 1.43

Ze 0.16

Did an R2 got 0.21 (Long lead) did not do a R1 + R2

As mentioned I do plan to look further, as the Ze was low.
I may find a loose connection and all is well when sorted.

But if I can't find anything loose to cause a higher resistance, how would you proceed. Would you start lifting boards etc?

 

[riveralt]

Sorry 220cm. re fan above shower


Looking at my test sheet now (was going from memory in first post) for circuit that caused concern

Zs 1.43

Ze 0.16

Did an R2 got 0.21 (Long lead) did not do a R1 + R2

As mentioned I do plan to look further, as the Ze was low.
I may find a loose connection and all is well when sorted.

But if I can't find anything loose to cause a higher resistance, how would you proceed. Would you start lifting boards etc?

You Ze isn't low - what are you talking about.
Have you done the proper tests for a ring final circuit?

If so what were your line to line measurements - you know r1, r2 and rn - which is different from R1 and R2.