Max Zs readings where circuits are protected by 30ma RCD's

[giio]

While I can't dispute your maths, the 1667 Ohm value is taken from Table 41.5 in BS7671:2008

your right there, but judging by the amount of amendments to the iee regs even before it comes out of print., you just never know,
in fact we are metaphorically walking a tightrope accross the Victoria falls with steel toecaps a safety helmet high viz and safety glassses on but without a safety rope a lot of the time, trying to do our best to interpret certain new changes whilst the IEE are still debating (for want of a better word) the merits or demerits as the case may be of the value of these changes.
rant over

 

[Spudnik]

30mA RCD;s are specified for ADDITIONAL protection on TN systems.

Therefore, Zs values should still be within the limits in BS7671.

 

[Deleted member 9648]

30mA RCD;s are specified for ADDITIONAL protection on TN systems.

Therefore, Zs values should still be within the limits in BS7671.[/QUOTE

An RCD may also be used for earth fault protection on a TN system.
411.4.4
531.3.1

 

[madmac]

I am in total agreement here, RCD's are for additional protection and the disconnection times should still be achieved and if they can't then further investigation is required to remedy the problem not just stick an RCD on the system and walk away quoting 1667 ohms!
On another related point here, I have mentioned the over reliance on RCD's on another thread and been shot down in flames by other members for suggesting sticking with EEBADS rather than blindly going with the 17th regs and only using ADS with RCD additional protection! IMO it's better to practice safety first and the 16th edition system gave you that extra earthing protection that would cover against an RCD failing.
I would also like to add that as I've said previously, these fast track courses don't help and neither does the regulatory bods who frequently turn a blind eye to qualifications held to be able to be let loose on the poor householder! Rant over.

 

[Deleted member 9648]

I am in total agreement here, RCD's are for additional protection...see regulations 411.4.4 and 531.3.1 and the disconnection times should still be achieved and if they can't then further investigation is required to remedy the problem not just stick an RCD on the system and walk away quoting 1667 ohms!As already stated,the 1667ohms figure is theoretical...a high measured Zs on a TN system would normally be as a result of another problem,which should be rectified before the Zs is even tested...high R1R2? or Ze?...or poor circuit design.
On another related point here, I have mentioned the over reliance on RCD's on another thread and been shot down in flames by other members for suggesting sticking with EEBADS rather than blindly going with the 17th regs and only using ADS with RCD additional protection!I dont see your point....Main bonding is still required regardless of RCD protection so as far as I'm concerned EEBADS is still in place,just re-labelled ADS. IMO it's better to practice safety first and the 16th edition system gave you that extra earthing protection that would cover against an RCD failing.What extra earthing protection?...if you are referring to supplementary bonding that is nothing to do with earthing....last time I looked at the 17th earthing had not been downgraded.
I would also like to add that as I've said previously, these fast track courses don't help and neither does the regulatory bods who frequently turn a blind eye to qualifications held to be able to be let loose on the poor householder! Rant over.

An RCD can be just a different type of earth fault protective device on a properly designed and safe circuit to enable disconnection times to be met...just as different overcurrent devices can be chosen to enable disconnection times to be met.

 

[madmac]

An RCD can be just a different type of earth fault protective device on a properly designed and safe circuit to enable disconnection times to be met...just as different overcurrent devices can be chosen to enable disconnection times to be met.

I'm just old fashioned though and don't like to solely rely on RCD's to meet disconection times, I think it's better to design the circuit to meet disc. times and only rely on RCD's as last resort but with 17th now requiring Rcd's to be fitted on systems not under maintenance or control of competent persons it leaves this dubious situation open for abuse.
You are actually agreeing with my first two points, but regards to EEBADS still being the same as ADS? I was trying to point out that 16th edition standards were good enough for nearly 20 years so why try and water it down and change industry standards like EEBADS to ADS where there is more reliance on individual pieces of kit(RCD) to protect the circuit rather than good design and meeting calculations for disc. times in the first place.
The whole issue of earthing and bonding has always been a tricky one to interperate and it doesn't help when the boffins writing these manuals keep moving the goal posts.

 

[baldsparkies]

Its a bit like listening to politicians or barristers from both sides of the fence.
One minute you think your thoughts are wrong, and then right, depending on how good the person is at putting there point of view.
If an RCD fails, the overcurrent device should still be able to meet earth fault disconnection times, end of.
And if TT or any other system cant do this, then they should be banned from use, and the supply authorities held accountable.
Electricity and the 1st edition came out in 1882. More than enough time to sort the situation out.
RCD's are about treating a situation that simply should not exist in 2011.
You can try and listen to the ifs whats and whys, but if your gut feeling says somethings not adding up !!!!!!

 

[IQ Electrical]

Its a bit like listening to politicians or barristers from both sides of the fence.
One minute you think your thoughts are wrong, and then right, depending on how good the person is at putting there point of view.
If an RCD fails, the overcurrent device should still be able to meet earth fault disconnection times, end of.
And if TT or any other system cant do this, then they should be banned from use, and the supply authorities held accountable.
Electricity and the 1st edition came out in 1882. More than enough time to sort the situation out.
RCD's are about treating a situation that simply should not exist in 2011.
You can try and listen to the ifs whats and whys, but if your gut feeling says somethings not adding up !!!!!!

With respect here, what exactly is this 'situation' that you are alluding to?

I have never ever in all of my time in this business (24 years) read or heard of a death attributed to a TT system or a failed RCD/RCBO and believe me, on electrical matters, my ear is kept 'to the ground'.

So please explain on what basis a TT system should be banned.

 

[baldsparkies]

With respect here, what exactly is this 'situation' that you are alluding to?

I have never ever in all of my time in this business (24 years) read or heard of a death attributed to a TT system or a failed RCD/RCBO and believe me, on electrical matters, my ear is kept 'to the ground'.

So please explain on what basis a TT system should be banned.

With respect IQ, in my 38 years in the business I have come across many failed ELCB's and or RCD's.
I have no doubt that many electricians on this forum have had similar experiances.
If you ask most home owners, when was the last time you pressed the little test button, you will usually get a blank look, and be told can't remember, or didn't know I had to, whats it for ??
And that's regardless of warning labels being present at the board.

TT has been around for many years, and almost always leaves an electrical system with high impedance earth return paths. This leaves the safety of the system very much reliant on the RCD.
I don't and never have liked the idea, its an opinion, its my opinion, and I'm probably not on my own with that opinion.
I bow to your technical knowledge, and have the greatest respect for you.
If we were Barristers in a court room, I am sure you could provide a far better argument than my humble self.
Its been done in the past, and murderer's have gone free as a result. Off topic I agree, put Im sure you take my point.

 

[Deleted member 9648]

I would agree on sole reliance on a single RCD on a high impedance TT system. But in my view the way to address that is to have a 100ma time delay main switch...and 30ma protection to final circuits.....it is highly unlikely that two RCD's would fail.Perhaps that is something which should be made a requirement of a TT install.
Regarding using an RCD on a TN system for earth fault protection...(the disputed 1667 ohms limit)...as already stated,on a properly designed and tested circuit which is reliant on an RCD to meet disconnection times because the zs exceeds the limit for the overcurrent device...(perhaps because a type c or d is to repalce a type b),the zs will only slightly exceed that maximum zs if R1+R2 and Ze readings are within acceptable limits. The overcurrent device therefore can still be expected to disconnect,perhaps just not in the required time.

 

[malcolmsanford]

If you ask most home owners, when was the last time you pressed the little test button, you will usually get a blank look, and be told can't remember, or didn't know I had to, whats it for ??
And that's regardless of warning labels being present at the board.

That is exactly the argument. Many electricians say they will not rely on an RCD, becasue of their failure rate, but as the ECA paper proved only about the 7% per cent of the tested units failed, of these, 5% was due to units being by passed and the other 2% had not been periodically tested as prescribed.

How often have we tested an RCD and it failed the first time, but after re- checking and operating the device, it conforms to the standard disconnection times. I always advised a customer of the test, even gave out leaflets on why it should be done, and you know full well that even before you have driven away, it will be ignored.

My arguement as always been, if you had a little button on your gas central heating boiler, and the gas plumber advised the customer

"Ok now every 3 months you have to press this button, ok it will turn the gas off, but if you don't then it could fail and cause either a fire or even kill you"

The customer would be pushing that button every few weeks let alone months, and that is becasue in the UK we think gas is more dangerous than electricity, and perhaps it is.

I think it is wrong of us to condemn a protection device that is not operated correctly. After all the BRB does mention manufacters instructions, which in 99% of RCD I would expect in the Uk is not adhered to.

 

[Deleted member 9648]

Trouble is pressing the test button only takes a second........closely followed by 3 hours going round resetting all the clocks and trying to get the alarm to stop bleeping....can see why they dont do it.

 

[baldsparkies]

On more than one occasion you can run a x1 followed by a x5 followed by a Bo##***s cus it now refuses to re set. Hello customer !! Now the thing is !!!!!

 

[Engineer54]

That is exactly the argument. Many electricians say they will not rely on an RCD, becasue of their failure rate, but as the ECA paper proved only about the 7% per cent of the tested units failed, of these, 5% was due to units being by passed and the other 2% had not been periodically tested as prescribed.

Sorry Malcolm, but i've read that report every which way and backwards, it's full of holes and it's test data is based on far too few examples to come out with the statements it makes to have any meaningful or justifiable conclusions. On my last project alone, we had more RCD device failures than the whole of the examples mentioned in this report...

To get an idea of the true failure rate, you only have to look in this, and other on-line Forums. they are an everyday occurrence!! The truth is, that today's RCD devices are not as fail safe as the manufacturers would have you believe. That's not to say that they are not improving all the time, it's just that there not quite there just yet!!! lol!!

 

[IQ Electrical]

Sorry Malcolm, but i've read that report every which way and backwards, it's full of holes and it's test data is based on far too few examples to come out with the statements it makes to have any meaningful or justifiable conclusions. On my last project alone, we had more RCD device failures than the whole of the examples mentioned in this report...

To get an idea of the true failure rate, you only have to look in this, and other on-line Forums. they are an everyday occurrence!! The truth is, that today's RCD devices are not as fail safe as the manufacturers would have you believe. That's not to say that they are not improving all the time, it's just that there not quite there just yet!!! lol!!

Of course there are failures, no product is perfect, but you can't use forum posts as an example, who joins a forum and posts about the longevity and reliability of their RCD/RCBO?

There are vastly more RCD/RCBO devices being installed now than at any other time due to the requirements of BS7671:2008 yet failure reports (even on forums) don't appear to have risen in proportion!

When was the last failed RCD/RCBO post here?

And more importantly, where is there a documented case of accident or injury attributed to a failed RCD/RCBO?