High zs reading on ring

[schoe]

Hi all did a zs reading on a ring circuit today and got a high reading of 1.26 on a 32 amp bs en 60898. I looked in the on site guide and it states the max is 1.16 but in the regs it says 1.44 so I don't no which one is correct?

When doing the zs by calculation zs= ze which is 0.18 tncs + r1 + r2 0.77 this is a spur the readings are fine.

Can anyone tell me why the zs would be high when carrying out the test manually?

Thanks Chris

 

[topquark]

Is there an RCD protecting the 32A MCB?

 

[schoe]

There is, but there also rcds protecting the other 14 circuits on the board and they all seem fine.

 

[telectrix]

if you are testing on no trip, the reading could be a bit inaccurate. the lower figure in the osg is to allow for temp. rise when cable is loaded. ( when warmer, the resistance is higher), but i can't see the cable running at 70deg.C. unless a really heavy load is in regular use. i always enter the 1.44ohms on test results.

 

[topquark]

Keep in mind that when you are testing Ze and R1+R2 and using that to calculate Zs that is not always the complete circuit. Any RCD protecting the circuits would not have been included (and have been known to add a considerable resistance the the complete circuit when testing).

As you've said the other circuits on that RCD are fine then most likely as Tel has said ^^^^^.

 

[schoe]

I'll re check all the tightness of the screws in the sockets anyway but would you say that this is an except able reading then?

 

[telectrix]

as it's below 1.44, the figure in the brb, then yes.

 

[topquark]

I'll re check all the tightness of the screws in the sockets anyway but would you say that this is an except able reading then?

Assuming this is for 2.5mm then yes, it's below the max. It can't be the screws in the sockets as that would have shown up in your R1+R2. It can only be down to additional resistance before the start of the RFC or as Tel has described (when you're using a no trip setting for your testing).

 

[ezzzekiel]

official figure is 1.44 but this must be adjusted for temp (this is the value given in the osg) rule of thumb method is 80% of total so:

max zs 1.44 x 0.80 = 1.16 so actual reading needs to be below this to comply

various things can effect the actual meter readings, often carrying out the same test at the same outlet will give varying results, providing you meet disconnection times all is well

 

[johnelectrical]

ezzzekiel is right the maximum is 1.16 this is because the fault current would cause the conductors to be hotter at the time of a fault, to allow for this extra current/heat/resistance you have to use the rule of thunb or figures which have already been adjusted in the OSG or GN3 not the 1,44 from the regs. as the circuit is protected byan RCD presumably 30mA the actual maximum Zs is 1,667. As for the difference between Ze + (R1 + R2) and the measured Zs, Zs Measured should always be less than the calculation, but iether the quality of the spur socket or the connection point at which you measured Ze and R1 + R2 are the points which need further investigation. if you put your Zs tester in the socket and do several tests you may well get a rang of different values depending on how you wiggle the tester leads connected to it

 

[needasparks]

That does seem a big difference from the calculated to the actual Zs obtained, are you sure it was not just an old worn socket, although you said it was from a spur off the ring i read it.