High Zs

[Rozz]

Is there anything I can do about a high Zs reading on a 3 bedroom house ring circuit. I've done some work in this property and I've extended the existing ring circuit. The property is approximately 20 years old and only has 1 ring circuit for the whole house which includes the kitchen. The fuse board has been upgraded to comply with the 17th edition Regs. and all the earth bonding is up to date. Most of the tests I have done are fine but the Zs for the ring circuit was 1.43ohms.

This circuit is on a 32A type B MCB and also protected by a 30MA RCD. Is there anything I can do to rectify the high Zs reading?

Please also note that the insulation resistance between N-E for this circuit was only 3.13Megohms and the Ze for the property was 0.58ohms if this helps answer my question. Many thanks.

 

[ricicle]

To be fair, if you are competant, you should know what things to look for if you believe any of your readings are suspect.

 

[securespark]

The high IR could be attributed to something still in circuit.

The Ze is high if it's a TN-C-S supply.

What are the ring continuity readings?

 

[Spark123]

A circuit with 30mA RCD protection can have a Zs up to 1666 ohms.
What is the Ze?

 

[bongos]

the Ze for the property was 0.58ohms if this helps answer my question.

I'm a bit confused though. I thought a B32 MCB had to have a Zs < 1.44&#937;

Or is this where 411.4.9 comes into play?

 

[RF Lighting]

1.16&#937; is the max Zs for a B32 according to my book.

You are allowed a maximum of 1666&#937; if the circuit is RCD protected, but it is always best to keep the Zs low enough for the OCPD to operate on an earth fault.

 

[Spark123]

the Ze for the property was 0.58ohms if this helps answer my question.

I'm a bit confused though. I thought a B32 MCB had to have a Zs < 1.44&#937;

Or is this where 411.4.9 comes into play?

1.44&#937; is a maximum tabulated value which applies to a conductor running at its maximum operating temperature.
80% of this is the maximum measured value (for a 70ºC conductor at 10ºC) which as Rob says is 1.16&#937;

 

[ericmark]

1.43 ohms is just within the limit for a B32 MCB so can't see problem.
230/1.43 = 160.6 amps for B type divide by 5 = 32.2 so OK.
But the Ze is more of worry as 0.35 ohms is max for TN-C-S so your reading a bit on high side so I would be reporting in case something is wrong with supply.

 

[Spark123]

I disagree - you are looking at the maximum tabulated value which is 230/(32x5)=1.44&#937;
The maximum measured value for a circuit consisting of 70ºC conductors measured at a conductor temp of 10ºC is 1.16&#937; (or 1.15&#937; if I work it out using 4 decimal places)
These maximum measured figures are published in GN3 and the on site guide.

0.58&#937; is OK as a Ze for TN-S, too high for a TN-CS and extremely good for a TT

 

[ericmark]

Although 200 ohms is considered OK when a RCD is used as any higher is likely to be unstable. This is for line v earth impedance and for line v neutral impedance you would still require 1.44 ohms if the B32 is going to work on the magnet circuit.

For the thermal part of the trip 7.1 ohms would make it trip but it would take a long time.

Not sure about volt drop considerations yet

 

[Spark123]

Yes, you will need 1.44&#937; or less for a B32 to operate on the magnetic trip.
As the conductor resistance increases with temperature, this 1.44&#937; needs to be when the conductor is at its maximum operating temperature hence the need to compensate for it when measuring with a conductor temperature lower than this.
The increase in temperature compensation factor (I've used F) can be calculated as follows:
F = 1 + (temp change * 0.004)
F = 1 + (60 * 0.004) = 1.24
So a conductor at 70ºC will have a resistance 1.24 x greater than it would have at 10ºC.

If you have a Zs of 1.43&#937; at 10ºC then when the circuit is under maximum loading so the conductor warms up to 70ºC then the Zs will also increase, as a worst case scenario to 1.77&#937; which is well above the maximum permitted value for a B32. (You could take Ze away from Zs, do the correction then add Ze back on at the end to be more precise).

To work the other way i.e. from 70ºC to 10ºC you can multiply by the reciprocal of 1.24 which is 0.8 ish or 80% ish.

 

[ericmark]

Working on 26 amp i.e. 20 at centre and 12 amp even distributed over circuit and allowing for earth being 1.5mm and line 2.5mm I get volt drop of 13.7 volts and maximum permitted is 11.5 volts.

As we reduce amps the 20 at centre and 12 amp even distributed over circuit goes out of the window so to get within allowed parameters maximum size of MCB will be a B20.

Sorry it took so long brain a little slow today.

I agree should use Ct=230+tp-(Ca2Cg2-Ib2/It2)(tp-30)/230+tp as Spark 123 has pointed out but unless the cable is really warm which is unlikely then it will not be correct anyway. Most cases we have not been drawing max just before the test. And if it complies with volt drop it is also likely to comply with the rest of the tests.

 

[Spark123]

That isn't the equation I was looking at, iirc that is something to do with volt drop when ambient temp is above 30ºC?? The one I was looking at is in appendix 2 of GN3, the 80% value is also skimmed across in Appendix 14 of BS7671:2008

 

[kevin1234]

1.43 ohms is just within the limit for a B32 MCB so can't see problem.
230/1.43 = 160.6 amps for B type divide by 5 = 32.2 so OK.

where are you getting 5 from i dont understand what is happening someone explain?

thanks

 

[securespark]

The high IR could be attributed to something still in circuit.

The Ze is high if it's a TN-C-S supply.

What are the ring continuity readings?

So, Rozz, is it TN-C-S or TN-S?

What are your ring cont. readings?