Max Ze for TT system

[Guest 004]

^ yea!!!!!

 

[Engineer54]

So, is your nightmare an illiterate Electrical Trainee, Part P 'registered' trying to 'do' a TT install

Please give people some credit - the wording is not that technical. There's a big difference between 'over 200 ohms can be unstable' and '200 ohms is the maximum permissible value'?

I'll leave the nightmares to the clever people like of you...

There are a good few real electricians out there that have never touched a TT system in their lives before, and as has been seen from previous threads and posts, that they will also take that 200 ohm figure as meaning the maximum. Like many other areas in BS 7671 things can be a tad on the ambiguous side, especially if you don't take the time to read things through to fully understand what's being stated.

 

[Guest 004]

my work is done, time to move to another thread

 

[D Skelton]

^ yea!!!!!

Before you get too happy Mr, just realise that the chances of a fault occuring that recreates the operation of the test button is impossible! lol

 

[D Skelton]

I'll leave the nightmares to the clever people like of you...

There are a good few real electricians out there that have never touched a TT system in their lives before, and as has been seen from previous threads and posts, that they will also take that 200 ohm figure as meaning the maximum. Like many other areas in BS 7671 things can be a tad on the ambiguous side, especially if you don't take the time to read things through to fully understand what's being stated.

Now that is a statement we can all agree on! lol

 

[i=p/u]

Does rcd not provide fault protection on said circuit with no cpc. Andy leakage and bang trips.

 

[D Skelton]

Does rcd not provide fault protection on said circuit with no cpc. Andy leakage and bang trips.

No, fault protection is provided by the class 2 accessories. The very nature of a class 2 installation is that there is no earth for current to leak to!

 

[spark 68]

I dunno Mr D, outside of the realms of our normal installations I could think of a few exotic cases where an RCD could be used that wouldn't use an earth per se, albeit the chances of most of us ever seeing it would be practically nil.

The nearest thing to this is our good old TNC-S system

 

[Engineer54]

Domestic wise however, I'd be happy with a couple of 4 footers, an Ra value of 200 ohms and a type-s up front. Last year I happily left a domestic with an Ra value of around 250 ohms even after punching in 8 foot of copper into solid chalk. I went back every month for six months in a row to check its stability and the value hadn't budged.

There are always going to be situations such as you have described due to horrendous soil/ground conditions, and you're solution of an upfront S type, backing up any standard 30mA RCD's is the route i would also take, in such circumstances...

I too have a good amount of chalky soil on my property in Cyprus, but thankfully not to the extent of causing me major problems on my own system...

 

[D Skelton]

I dunno Mr D, outside of the realms of our normal installations I could think of a few exotic cases where an RCD could be used that wouldn't use an earth per se

Like what?

 

[spark 68]

Like what?

Certain IT systems use some pretty exotic protection schemas, also some DC systems are another, and I believe someone on here a while back came up with an idea using one (un-conventionally) to protect against a lost Neutral.

But as I said this is way off the beaten track, and is sidetracking this interesting thread, and almost all of us will never ever see it, it was more of an answer to Mr E, and partly the way the test button works (which is why I mentioned TNC-S).
I will not mention this part again, let the thread continue un-abated

 

[D Skelton]

Certain IT systems use some pretty exotic protection schemas

Every IT system that incorporates an RCD will have an earth sensing device to allow the RCD to operate. Although not an earth per se, it still requires a means to detect an inbalance.

also some DC systems are another

Even a DC RCD needs a means of detecting imbalance between pos and neg which isn't going to happen unless a fault loop has been intentionally made at the supply end. Again, functional imbalance detection would need to be employed.

I will not mention this part again, let the thread continue un-abated

Agreed

 

[MBEINST]

An update on the situation with my TT system.

I today called the DNO 'Electricity North West' and explained that I was at a property to install a couple of extra sockets on the downstairs ring and my Zs readings were unacceptable. I requested that the TT system be changed to TNCS since my subsequent Ze readings mean that no matter what I do to the householders installation I cannot get acceptable Zs readings (OSG Table 2d). I also mentioned that the occupants are in fact living with a dangerous installation since they effectively have little or no fault protection.

The nice man at the other end was quite alarmed and told me that an engineer would be with us before the day is out. Unfortunately this turned into tomorrow morning. However the earthing arrangement will be changed free of charge.

This appears to have bolstered my credibility with the householder and he now wants me to install a fully loaded RCBO unit.

I have returned with this update to thank you all for your input and advice. You have succeeded in enlightening a raw but 2391 trained spark (not sure if that qualifies me as a microwave spark but certainly not a DIYer). I like a little banter myself but some of the drivel in this thread boarders on people stuck up their own arses trying to score cheap points. Tone it down guys, its not big and its not clever.

Special thanks to Wirepuller and Engineer 54.

Everyday is a school day!

 

[D Skelton]

I like a little banter myself but some of the drivel in this thread boarders on people stuck up their own arses trying to score cheap points. Tone it down guys, its not big and its not clever.

I can't see any drivel on here, everyone who has commented has a fair amount of knowledge on TT systems. I can assure you that what you might think is point scoring is purely banter.

Special thanks to Wirepuller and Engineer 54.

Interesting how you give special thanks to the two very people that most on here would regard as being at complete opposite ends of the spectrum with regards to their opinions on domestic TT systems yet refer to others opinions as drivel. I find the term drivel quite disparaging if I'm honest, I can't think of anyone here who hasn't contributed in a positive way to the thread?

 

[MBEINST]

What was it you said:

"I got a bite! A big one! C'mon lads, get the landing net out ready, this one's gonna be a wriggler!"

Sorry that I'm not interested in winning popularity contests and taking sides in petty arguements.

I thanked everyone on the thread especially those 'wirepuller' & 'Engineer 54' whose advice I followed and which resolved my situation and in fact got me extra work.

However if this leaves you feeling a little under appreciated:

A great big extra special thank you very much to:

D Skelton :broken_heart: :devilish: