Zs on outside light, can some one please help?

[Ross1]

Morning Guys,

Ive done a Zs test on an outside light which is on its own circuit and am getting a reading of 35.8 ohms on no trip mode. Im using a fluke 1653 multimeter which im new to.
In the bottom of the display below the Zs reading, its displaying Ik 7A. My Ze is 75.4 ohms.

Am I correct in using the formula Zs x Ia must be smaller than or equal to the Line Voltage to earth.

which would be 35.8 x 7 = 250.6V

Its a TT installation

 

[malcolmsanford]

The Ra (Ze) for the system is ...............................

The R1 + R2 for the circuit is ........................

Zs = Ze + ( R1 + R2)

As you know, and the reason we apply additional protection for especially TT systems is because of what you are finding, high Zs readings, normally due to a poor Ra. (Ze)

A 30mA RCD Zs is 1667ohms so your 35 ohms is perfectly ok to trip that RCD. If I were you I would now check the Ra (Ze) of the rod and see what that is. If it is high, which I expect it is then you need to advise your client that you recommend reducing the value.

also do a R1 + R2 test and see if this is ok, if it is high then you will have a loose connection somewhere

 

[cjp1]

Ze is 75.4 & Zs is 35.8 (Zs probably lower than Ze + R1 + R2 due to parallel earth paths)
As Malcom said as long as you have confirmed continuity of cpc (R1+R2) then should be fine.

 

[Ross1]

The Ra (Ze) for the system is ...............................

The R1 + R2 for the circuit is ........................

Zs = Ze + ( R1 + R2)

As you know, and the reason we apply additional protection for especially TT systems is because of what you are finding, high Zs readings, normally due to a poor Ra. (Ze)

A 30mA RCD Zs is 1667ohms so your 35 ohms is perfectly ok to trip that RCD. If I were you I would now check the Ra (Ze) of the rod and see what that is. If it is high, which I expect it is then you need to advise your client that you recommend reducing the value.

also do a R1 + R2 test and see if this is ok, if it is high then you will have a loose connection somewhere

R1 + R2 is 0.30 ohms

 

[Ross1]

There is a 10mm earth going to the Rod so Ze not gonna change significantly is it guys.

 

[malcolmsanford]

The CSA of the earthing conductor will have no great bearing on the value of your Ra, you could put a 240mm or a 2.5mm conductor and the results, unless it's a great distance will be virtually the same, you need to test the Ra of the electrode to ensure what it's value is.

 

[cjp1]

Your Ze reading for a TT system of 75.4 is fine.

 

[Ross1]

The CSA of the earthing conductor will have no great bearing on the value of your Ra, you could put a 240mm or a 2.5mm conductor and the results, unless it's a great distance will be virtually the same, you need to test the Ra of the electrode to ensure what it's value is.

When you say test the Ra malcolmsanford I take it you mean an earth electrode resistance test where you knock in 2 test spikes?

 

[Ross1]

Your Ze reading for a TT system of 75.4 is fine.

I beleive NIC like it to be below 100 ohms, was concerned about the high Zs for the circuit

 

[malcolmsanford]

That is one way of doing it yes, but you can do a Ze which is what you have done and got 75 ohms and that is within the Regs guidelines of 200ohms and the BS 7430 of under 100ohms.

As both stated cjp1 and I stated because of the 30ma RCD protection you will fit the Zs of 35 ohm is well withing the required 1667ohms.

You have noted that your R1+R2 id 0.30ohms and that seems fine.

 

[malcolmsanford]

I beleive NIC like it to be below 100 ohms, was concerned about the high Zs for the circuit

Ross as I said in post #2 it is the reason that TT systems, even before the 17th edition, had RCD and before these ELCB protection because of the fact that you will get a higher than normal Zs value, and disconnection times for the final circuits was often difficult to achieve.

 

[Ross1]

That is one way of doing it yes, but you can do a Ze which is what you have done and got 75 ohms and that is within the Regs guidelines of 200ohms and the BS 7430 of under 100ohms.

As both stated cjp1 and I stated because of the 30ma RCD protection you will fit the Zs of 35 ohm is well withing the required 1667ohms.

You have noted that your R1+R2 id 0.30ohms and that seems fine.

ok cheers , Got it now.