maximum zs

[Andy B]

What table do I use in the on site guide to find the maximum Zs for a TT sytems? My Ze is 3.61 so they all be over anyway?

 

[telectrix]

for a TT system your max. Zs is 1667. end of.

 

[VoltzElectrical]

on a tt system, ze is acceptable up to 1667 ohms because you are obliged to put everything on an rcd.

 

[VoltzElectrical]

lightning fingers tel, you beat me!

 

[telectrix]

and rcd disconnection time is 0.2 sec.not 0.3.

 

[scotsparky]

Hi Andy

As the consumer has to supply an earth in a TT there is huge changes in Ze valus. If you have a look at the regs there is a section on TT systems in section 4 and it explains it. A general rule is you Ze must be under 200ohms so 3.61 is great
As there is such uncertanty RCDs must be used. The maximum ZS for a 30ma RCD which has to be used for a final circuit is 1667 ohms.

Th OSG says this but its much more detailed in the BRB

 

[Andy B]

Thanks guys. On the test sheet for the maximum zs i put 1667 ohms for every circuit.
I`ve got my part p assessment tomorrow. I`m doing the test sheet for job I`m using.

 

[Andy B]

I was unable to do a Zs on the upstairs lights. As soon as I touched the red on the live the circuit tripped?

 

[Deleted member 9648]

Calculate it......Ze+R1R2..

 

[telectrix]

happens. if you have an R1+R2 reading, add to the Ze to give a calculated value.

 

[Deleted member 26818]

To be pedantic the maximum Zs allowed on a TT system, is not necessarily 1667Ω.
That figure only relates to wher 30mA RCD protection is installed.
Where 100mA RCD protection isinstalled, 500Ω is the maximum.

 

[Andy B]

ok so shall i put 1667 on the sheet for every circuit?
Thanks
Andy

 

[TPES]

ok so shall i put 1667 on the sheet for every circuit?
Thanks
Andy

Yes your fine to put that in for each circuit, max zs for circuits protected by 30mA RCD protection is 1667ohms TT or TN.

 

[Deleted member 26818]

ok so shall i put 1667 on the sheet for every circuit?
Thanks
Andy

Only on those circuits protected by a 30mA RCD.

 

[scotsparky]

To be pedantic the maximum Zs allowed on a TT system, is not necessarily 1667Ω.
That figure only relates to wher 30mA RCD protection is installed.
Where 100mA RCD protection isinstalled, 500Ω is the maximum.

very true spin but you have to put in a 30ma RCd in for final circuits and it is the end of the final circuit you take te ZS from which is 1667 ohms