ZS readings on test report TT system

You are meant to put the maximum permissible, which on a TT, will vary depending on the RCD. For example, for a 30mA RCD, you could go upto 1666ohm (50v touch voltage devided by 0.03amps).

 

1666 ohms where it asks for maximum Zs allowed by BS7671 and put the measured figures in the Zs boxes.

 

If you are exceeding the Zs readings for individual ccts, they need to be investigated.IMO
Ensure you are not taking main bonding into account and purely confirming the integrity of the earth electrode itself.
100 Ohms is the recommended limit on TT systems(for rod only...no bonding) but as Sen says the theoretical limit is 1666.
RS

 

It's the MAX bs7671 value you're recording on the cert, not the value you would be expecting.

 

Which on a TT system with 30m/a rcd would be 1666 ohms.

 

Which is exactly what I put on my post further up. I was just clarifying it as Retiredsparks was saying it needs investigating if you measure this value.

 

It asks for the max Zs for the MCB though, not the RCD, unless its RCBOs. 99% of MCBs are covered by an RCD nowadays, even on TNC-S it could be 1667 ohms and be safe(ish). I have always put the max for the breaker in that column, if any clever dick is ever clever enough to spot the anomaly on the sheet then you can explain all about RCDs to him in your best adenoids voice.

 

I always used to put the max value for the MCB, and had done for years. On NIC inspections, it was always fine. Recently, the NIC have said they want you to put the "maximum allowable value for BS7671", which is the RCD max.

 

I am still open to suggestions on this particular issue. The maximum allowable figure of 1667 ohms is meaningless. It is the limiting value in ohms that a 30mA RCD will trip at so that 50V is not exceeded.

All values of impedance LOWER than 1667 ohms are safe. The limiting factor for TT system earth is not the maximum theoretical reading in ohms to satisfy the less than 50V equation but the stability of the rod. If a TT system is delivering over 200 ohms then it is UNSTABLE. The maximum desireable reading on a rod is 100 ohms for it to be STABLE. In reality, unless there are factors causing higher readings such as ground conditions or loose earth connections on the rod, then TT Ze measured readings are generally lower than 100 ohms. It is all very well putting 1667 ohms as the maximum allowable reading but I disagree that it is. 100 ohms is the maximum for reasons to do with suitable rod readings, not the 50V equation. We know that as long as the Ze is less than 1667 ohms, and lets face it it always will be, then we need to consider maximums for other reasons. Quite frankly recording 1667 ohms is ridiculous.

 

Funny that Lec because I always put 1666 but on my recent assessment he picked me up on it and said that wasn't right. He didn't make much of an issue over it though and im still going to carry on doing it the way I always have done! Maximum Zs allowed by BS7671 when there is a 30m/a rcd in circuit is 1666 ohms!

 

Personally I can't see that Zs readings on a TT earth system have any relevance. The maximums quoted in tables are relevant only to TN systems. Recording 1667 ohms is not relevant either. Zs is an earth fault loop. TT just doesn't have earth loops low enough. The Ze will always be higher than all the tabulated values and so will make Zs pointless. What is the relevance of recording any Zs values at all?

 

That's exactly why the NIC are saying to input the maximum of 1667 and not the tabulated 6.something for the B6 MCB. Your reading is going to exceed the tabulated maximum, but there still IS a maximum is BS7671 of 1667.

 

It does clearly say on the certs "maximum Zs permitted by BS7671". If you are relying on an RCD for disconnection, you should be entering the maximum Zs allowed for the RCD.

 

Well, if it's > 1666 ohms on a 30mA RCD then you know that you'll get a touch voltage more than 50v, which may not be healthy.

But if you've got a TT submain in SWA, for instance, and it's protected by a 300mA RCD, then the Zs max permissible shouldn't be more than 166.6 ohms. And you'd really want to be looking at a much lower Zs than that, so you'd bang more rods in.

And a 500mA RCd (for fire protection in a TT system, for instance) would be 100 ohms max.

So the reading for Zs in TT systems then becomes more relevant.

 

I don't have the Regs book on me either, but 230 / 6 / 5 = 7.67 Ohms so that is the maximum for a 6A Type B BS EN 60898 circuit breaker (not allowing for Cmin factor as BS 7671:2008 (2013) is still current).

 

In theory, theory and practice are the same. In practice, they are not.”

― Albert Einstein

Would any of your guys be happy with a Zs on a ring cct being 365 ohms in your own homes ?

RS

 

No I wouldn't RS and I wouldn't leave anyone else's like that either! I remember putting 2 rods in on a job several years ago which got the Ra down to around 80 ohms, the best I could get with a single rod was about 50 ohms from memory.