Slightly High Zs

[PaulSC]

Evening all,

Just been to look at a CU change job today and took a quick Zs reading on a number of socket outlets (only one ring final circuit). On a few of the sockets, I got a reading slightly above (1.6 ohms) the maximum recommended in the OSG for what will be 60898 MCB's. Am I right in thinking as this is to be a 17th edition board change (dual RCD) then anything below 1667 ohms is still acceptable, though not ideal.

Customer wants all new accessories, so these readings may well come down, especially in the kitchen where years of fat, steam and crud may have increased the resistance of the socket (readings taken at the front of sockets with 3 pin adaptor for Megger).

Thanks

Paul

 

[batty]

Could be a lot of things worn sockets, Extended ring what was the earthing type? If its a TN system I would want to ensure readings complied.

 

[markyspark76]

If you have a B32 mcb if Zs has been exceeded greater than 80% value of 1.44ohms you have to do a live neutral loop reading.if this is less than 1.44ohms then it will disconnect in less than 0.4 sec under short circuit conditions

 

[sparkytim]

But as you rightly say 30 m A RCD protection does not depend on low Zs for shock protection like MCBs.

 

[apprentice87]

Hi,

Makes no difference how fast it disconnects if you have a phase/neutral fault, this is not the issue, it is how fast it disconnects under earth fault conditions that the regs are concerned about......

Besides, if you did actually have a phase/neutral loop impedance of 1.44 ohms, then you would find that once you applied a load of about 9 amps you would have excessive volt drop anyway....

Still, if on a TN system, you cannot meet the required disconnection times by using using an overcurrent device on its own, [Zs too high] then yes, you can use an RCD IN ADDITION to achieve this, as you obviously need overcurrent protection too.

john....

 

[speedster]

Interesting one.

In theory, the RCD protecting a circuit will ensure disconnection within required time.

You mention one ring circuit.

Is this an installation with just one ring circuit supplying all sockets ?

I am wondering just how long the total ring circuit is.

Obviously the longer the circuit the higher the impedance !

I would check the Ze,and also do the standard ring circuit checks . This may give you some pointers.

It may be some of the connections are a bit loose.

If there is only one ring circuit supplying the whole property,maybe it's time for an additional ring and or some major alterations.

Best wishes,

Speedster.