Regs on a high Zs

[Booyaka]

Hello i just wanted to run something by you..

if you had a high Zs reading on circuit on a TT system but the TT system is ok on Ze (5 ohms) does the regs state that if the circuit is protected by an RCD then it can be waivered?

many thanks

 

[ricicle]

Yes the prefered method is to use an RCD for high Zs on a TT system and anyhow all socket outlets must be protected by an RCD on this system.

 

[Booyaka]

many thanks that confirmed what i was thinking, they are protected by an RCD so its all cool

cheers

 

[unclebubble]

can I ask mate what readings did you actually get or did your tester keep readin 2000 ohms.I'm asking this cos I had the same problem 2 weeks ago and the NICEIC are saying I must have a reading that conforms with regs!Cheers

 

[DESL]

How high was the Zs was it much higher than (R1+R2) + Ze ?

 

[RF Lighting]

Are you sure your tester wasn't displaying >2000 ohms?


Quite right that they are insting that your Ze complies with BS7671.

Do you understand the consequences of not having a low enough Ze?

 

[Steve3948]

From memory if a Rcd is used the reading can be up to 999 ohms.

 

[GaryMo]

Picked this off another forum:

"The requirement for Zs is then just to be low enough for the RCD to trip before raising the metalwork above 50V (Zs <= 50V / In - i.e. 500 ohms for a 100mA RCD, 1,666 ohms for a 30mA one) "

 

[Steve3948]

Picked this off another forum:

"The requirement for Zs is then just to be low enough for the RCD to trip before raising the metalwork above 50V (Zs <= 50V / In - i.e. 500 ohms for a 100mA RCD, 1,666 ohms for a 30mA one) "

Gary interesting find that, I once contacted Megger as I was concerned how high my Zs reading was on a 30mA using the No Trip Test mode, when I first purchased the machine.
The tech guy said that any reading to 999 ohms would satisfy the requirements for the use of a 30mA Rcd.
I did mention it was for an NIC certificate, so I wonder if this is another of the NIC pet requirements, if it is in fact 1,666 ohms.

 

[ricicle]

Although with a 30mA RCD the equation says that the Ze must not be higher than 1666 ohms, guidance says that anything above 200 ohms can be unstable and I believe the NIC require the reading to be sub 100 ohms.

The touch voltage on construction sites, agricultural/horticultural installations must not rise above 25V.

 

[securespark]

Surely if your Ze is 5 Ohm, your Zs should not be heaps greater?

 

[DESL]

SS I think this thread has gone slightly off OPs question - see my reply.

 

[Booyaka]

Hi Guys

My Zs reading was around 8ohms and my Ze was 5ohms for definate as i retested a few times, now as the outbuilding is protected by a 30mA RCD then i am lead to believe this is fine and complies with the regs

 

[securespark]

SS I think this thread has gone slightly off OPs question - see my reply.

I'm always late....Sorry!

 

[Spark123]

Hi Guys

My Zs reading was around 8ohms and my Ze was 5ohms for definate as i retested a few times, now as the outbuilding is protected by a 30mA RCD then i am lead to believe this is fine and complies with the regs

So you have 3 ohms of internal resistance? On a submain? May be a little bit high. What size/length of cable? What are your R1+R2 readings like?
In a TT system RCDs are used to provide protection against indirect contact. You also need to remember, 30mA RCD for supplementary protection against direct contact for all sockets reasonably expected to supply portable equipment outdoors etc.