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?

 

[Engineer54]

Ive read about ufer earthing, and it sounds interesing to say the least. Im going to speak to our electrical engineer about it to see if it can be applied to some of our new sites that are having civils done at the same time.

Even ufer earthing arrangements benifit greatly from installing a couple of earth rod positions at opposing corners and connected to the slab re-bar via exothermic/braise welding.


It certainly makes sense to use what's already there, i'm surprised that much more use isn't being made of ufer type earthing, no matter what type of earthing arrangement you currently have. It can be linked to any form of earthing MET....


Malcolm,
I don't allow those clamps, they have a distinct tendency to lose there grip in a very short time, and are almost useless when using lugs of any size, like the one shown in your photo, having hardly any metal to metal contact area. I tend to use Furse types of connections, or substantial ''U'' Bolt connections...

 

[chronyx]

heres an example if no RCD is provided and there are high ZS readings due to a Ze of 30 ohms due to the earth rod how are you going to satisfy disconnection times, prove in the event of fault theat any voltage on exposed conductive parts is under 50v?

Evening all, hope you're having a good Easter break. No rest for the wicked so I've been making the most of the dire weather and working. Found this thread while looking for an answer on Google.

I'm doing a EICR on a 1930's bungalow with a TT system. Bit of a disaster area but the Ze is 15 ohms which is reasonable. However I'm struggling to find maximum Zs values for a TT system with no RCD protection. This installation only has 60898 MCBs.

Any more thoughts on this?

Cheers!

 

[scotsparky]

you dont get max Ze values for a TT system with no rcd protection.
If the building has no RCD protection and relys on 60898 mcab only you mill have overcurrent protection only but no fault protection. You will not get your disconnection times under fault conditions with a ZE of 15ohms

 

[chronyx]

Thanks scotsparky. That's exactly what I thought. It's an old Protek board and RCBO's will be either non-existant or too expensive. The whole place needs re-wiring really but a board change minimum. Although I then become liable for all the existing circuits.

I hate giving people bad news, the place is damp and in serious need of a lot of TLC is it is. The surveyor's report isn't worth the paper it's printed on.

Previous owner - a builder.

 

[telectrix]

but your max. Zs figures are still related to the MCB e.g. 1.44 for a 32A type B, so all will fail. the 1667 value is only if you have a 30mA RCD fitted.

 

[chronyx]

but your max. Zs figures are still related to the MCB e.g. 1.44 for a 32A type B, so all will fail. the 1667 value is only if you have a 30mA RCD fitted.

I thought that was only for TN systems, but I'm a little tired to get my head in the BRB now.

 

[scotsparky]

Zs values are for fault protection

 

[IQ Electrical]

I thought that was only for TN systems, but I'm a little tired to get my head in the BRB now.

Good answers above but you really need to brush up on the reg's if you're doing EICR's-this is quite basic stuff and your client is paying for your expertise and advice.

 

[telectrix]

at the very least, advise an up front RCD be fitted .

 

[Guitarist]

Personally, I'd be strongly advising that a new CU is installed, providing that the wiring itself is acceptable, as technically we shouldn't be installing an up-front RCD to cover an entire installation. You will not be taking responsibility for the whole installation by doing so, you will merely be recording that the circuits you reconnect test safe for continued use.

 

[chronyx]

Cheers lads!

at the very least, advise an up front RCD be fitted .

I tried to get hold of an RCD main switch as a cheap workaround while they consider a re-wire, but the L/N terminals are reversed on the T100-30-2 I bought, so that's no good. (Thank you Protek technical helpline, I hope my wholesaler will do a return)

I know it breaks the 'no circuit should interfere' reg but frankly it's still safer than no RCD at all. The house I live in now has an RCD main switch from the late 80's and we're all still alive nearly 20 years after moving in!

Good answers above but you really need to brush up on the reg's if you're doing EICR's-this is quite basic stuff and your client is paying for your expertise and advice.

I see your point, although, I passed the last of the 2391 exams back in March last year. I'd rather be aware I don't know or need to revise something than blindly assuming I still know all I learnt over a year ago!

Personally, I'd be strongly advising that a new CU is installed, providing that the wiring itself is acceptable, as technically we shouldn't be installing an up-front RCD to cover an entire installation. You will not be taking responsibility for the whole installation by doing so, you will merely be recording that the circuits you reconnect test safe for continued use.

The circuits are a mixed bag. Lighting seems to be really hacked about, most ring mains are fine although one has a lack of continuity on the neutral. Power to an outside shed without it's own earth rod. Downlight connections in choc block with no enclosure. The whole place has had half-arsed work done and not been looked after.

 

[Guitarist]

Just condemn it then, if it's that bad. I totally agree that an up-front RCD would make it much safer, but from what you've said, it's unlikely to hold anyway. Also, it would be in breach of regs.
Personally, from what you've described I wouldn't touch this system at all until I had permission to sort it out properly. An up-front RCD would simply give the owner the excuse that "it's ok now, as it's protected."

 

[chronyx]

I hate giving people bad news! It's pretty dire - maybe kindly described as serviceable but inherently unsafe.

Thanks for your help everyone, just typing up the EICR now.

 

[applemac]

It's not bad news it's just life, an installation only lasts a period of time fact. If someone buys a property then they should factor in upgrade costs. Nice new board, rewire, good earthing, nice safe job done.

 

[chronyx]

Nicely put applemac