Higher than anticipated Zs

[dizz]

I measured my Ze by disconnecting earth and testing between Earth and Live at the CU = 0.12 ohms.

I previously have done all the R1+R2, and in particular have a reading of 0.18 ohms on a short radial, and 0.21 ohms on the ring. These tie up with what I should expect to see when comparing the resistance per metre for the csa of the cable, so am satisfied that these are correct.

So Zs calculated would be 0.30 for the radial and 0.33 for the ring, but I am getting readings using the NO TRIP option on the Megger of 0.66 ohms fior the radial and 0.75 ohms on the furthest ring socket. While these are well within the limits, why is it higher?

Any takers?

 

[ricicle]

The 'no trip' Zs readings on the Megger are far from accurate !

 

[ericmark]

Subtract the incomers loop impedance from your readings. Typical 0.35 so you should expect 0.53 and 0.56 this assumes the Ze is 0.35 I know the DNO workers will turn a blind eye to 0.4 so it could be higher.

 

[ricicle]

Subtract the incomers loop impedance from your readings. Typical 0.35 so you should expect 0.53 and 0.56 this assumes the Ze is 0.35 I know the DNO workers will turn a blind eye to 0.4 so it could be higher.

Eric that is the most meaningless comment you have ever posted on here. I gave the answer in the first post. There is no 'typical' Ze reading.

 

[dizz]

So, is it acceptable to add my 0.12 ohms to each of the R1+R2 I measured to get the implicit Zs for each circuit?

Also, what the hell is the point of the Megger offering a no Trip Zs if it doesn't work?

 

[flameport]

Also, what the hell is the point of the Megger offering a no Trip Zs if it doesn't work?

So they can charge more money for the device.
That function would only confirm an already known fact even if it worked properly.

 

[17thman]

are you measuring Zs at the socket outlets with a socket and see? and measuring R1 + Rs with the probes?

 

[thomo27]

So, is it acceptable to add my 0.12 ohms to each of the R1+R2 I measured to get the implicit Zs for each circuit?

Also, what the hell is the point of the Megger offering a no Trip Zs if it doesn't work?

Yes, the NIC recommend this method to find ZS on Lights , spurs etc rather than test live by putting your probes onto Live and Earth, obvious different for ZS on sockets..

 

[17thman]

So, is it acceptable to add my 0.12 ohms to each of the R1+R2 I measured to get the implicit Zs for each circuit?

Also, what the hell is the point of the Megger offering a no Trip Zs if it doesn't work?

Yes, the NIC recommend this method to find ZS on Lights , spurs etc rather than test live by putting your probes onto Live and Earth, obvious different for ZS on sockets..

that is very helpful considering that he is installing and testing socket outlets.

 

[securespark]

Now, now...

It's helpful information nonetheless...

FWIW, my Alphatek seems to give a much higher reading on nontrip setting too.

 

[thomo27]

So, is it acceptable to add my 0.12 ohms to each of the R1+R2 I measured to get the implicit Zs for each circuit?

Also, what the hell is the point of the Megger offering a no Trip Zs if it doesn't work?

Yes, the NIC recommend this method to find ZS on Lights , spurs etc rather than test live by putting your probes onto Live and Earth, obvious different for ZS on sockets..

that is very helpful considering that he is installing and testing socket outlets.

OK dick chill your boots

 

[thomo27]

edit

 

[dizz]

are you measuring Zs at the socket outlets with a socket and see? and measuring R1 + Rs with the probes?

Yup - with the probes at the socket, and as the radial is pre-wired conduit, L, N and E are all 2.5mm, and I am getting exactly 0.18 ohms for L-N, L-E and N-L.

Yes, socket plug in Megger, tested four different sockets, each roughly the same.

 

[thomo27]

Now, now...

It's helpful information nonetheless...

thankyou

 

[dizz]

I suppose we are adding in RCD's and MCB's into the equation when doing the Zs?