TT earth systems versus TN earth systems

[unphased]

I was prompted to post this after reading the post started by electrcian786 British Gas/ DNO problem.

TN earth systems give earth fault loops below 1 ohm (Ze). TT earth systems fault loop is over 1 ohm (Ra). I am deliberately avoiding quoting the typical values because it is not critical to my post.

Fault disconnection times in the regs are reliant on keeping within max Zs values. TN systems Ze are generally low enough to allow earth fault loops to be within the tabulated values for the final circuits. TT systems will generally push Zs over tabulated max Zs values.

Now to the point of my post.

I am specifically referring to EARTH fault loops here as faults within the lines are not affected by earth impedance (overload or short circuit). In order to achieve disconnection of a fault to earth on a TT system an RCD is needed. Without an RCD the earth fault loop impedance will be too high to rely on the fuse to disconnect the fault in time. This raises several questions that confuse me.

DNOs appear to be getting complacent about providing an earth and are inconsistent in how they respond to requests for earth systems to be installed or even verified.

DNOs appear to be reluctant to install earth systems and are more than accepting for a customer to install their own earth rod.

On the face of it, TN systems under previous editions of the regs, where split load fuseboards were the default choice, relied on good TN systems to be in place, else the safety of non-RCD protected circuits would be compromised under earth fault conditions.

So, given the advent of 17th edition and the total RCD protection (30mA) of all circuits, do we really need TN earths any longer? Should we be TT by default? Am I missing something?

Just to be clear for all those students reading this thread and be certain that this is not a talkdomestic electrics only thread:

In DOMESTIC installations is a TT earth any safer or less so than aTN?

In DOMESTIC situations, EXCEPT WHERE A REG OR MANUFACURER of an item or accessory specifically prohibits, we can adopt 30mA protection on every circuit...

 

[OnOff]

So, where the pikeys are thieving the cables and just in case the DNO effs up (and it's seeming more common)...........TT with DP RCBO's as standard it is then and do away with all those nasty split load boards!

 

[Doc Hudson]

So, given the advent of 17th edition and the total RCD protection (30mA) of all circuits, do we really need TN earths any longer? Should we be TT by default? Am I missing something?

This is a mistaken assumption that probably less experienced electricians still make and will no doubt become another electrical myth probably generated from the abundance of short course training providers. To just clarify; There is NO requirement under 17th edition for 30ma RCD protection to be installed on all circuits.

Doc H.

 

[m4tty]

I see what your saying doc but also see when up is coming from. Tbh most circuits are chased in wall less than 50mm so majority of ccts need rcd protection.

Tbh whenever I come across a property with no earth I call the dno and tell the customer it's

 

[unphased]

This is a mistaken assumption that probably less experienced electricians still make and will no doubt become another electrical myth probably generated from the abundance of short course training providers. To just clarify; There is NO requirement under 17th edition for 30ma RCD protection to be installed on all circuits.Doc H.

With respect. This is not a mistake. It is far easier to install an RCD on a circuit and know it will comply than to spend ages justifying why there isn't. I have no idea why electricians spend so much time trying to prove that an RCD is not becessary. Just put one on all circuits and safety is improved. This is largely for domestic circuits any way, I don't dispute that RCDs on industrila or commercial circuits are less of an issue on all circuits. Just to clarify I know there is no requirement for all circuits to be protected by a 30mA RCD.

Getting back to the original post, what are your thoughts on the use of TT systems?

 

[steptoe]

I like TT.

Further to just fitting a 30mA RCD, as per a recent thread, certain pieces of equipment specifically phrohibit 30mA RCD, SMA inverter as an example.

 

[Doc Hudson]

As the forum is not the Talk.DOMESTICelectricinsforum, it is all areas of electrical work, so it can be best to be clear about what areas you are describing to avoid confusion. The 17th edition does not require RCD protection on all circuits, so your statement is factually incorrect and could give the wrong impression to students reading the forum. This is the reason why where necessary I will try to ensure that a balancing view is posted and reduce furthering the myths that this industry seems to absorb. Just for the record there are situations what it is not appropriate to install overload protection either so a stick an RCD on everything can be more of a danger than a safety issue. Your opening post made no mention of Domestic only environments, so it can be best to ensure these points are clarified.

Doc H.

 

[m4tty]

As the forum is not the Talk.DOMESTICelectricinsforum, it is all areas of electrical work, so it can be best to be clear about what areas you are describing to avoid confusion. The 17th edition does not require RCD protection on all circuits, so your statement is factually incorrect and could give the wrong impression to students reading the forum. This is the reason why where necessary I will try to ensure that a balancing view is posted and reduce furthering the myths that this industry seems to absorb. Just for the record there are situations what it is not appropriate to install overload protection either so a stick an RCD on everything can be more of a danger than a safety issue. Your opening post made no mention of Domestic only environments, so it can be best to ensure these points are clarified. Doc H.

Hi doc,

Can you clarify the last sentence please. (not picking just learning) as I believed rcd's do not offer any overload protection.

Thanks mate

Matt

 

[ParkySparky]

Matt this is correct, I feel you need an RCBO for overload protection aswell as earth leakage?

An RCD is Residual current device,

So an rccb can be fitted which is a residual-current circuit breaker which only offers earth leakage protection not short circuit or overload.

Or a Residual-current circuit breaker with overload protection can be used, a RCBO. This is what is used mainly in CU's domestically.

An RCD is a generic term for both of above, or you can fit individual sockets with their own rcd which can be passive or active.

Hope this helps.

 

[Doc Hudson]

Hi doc,Can you clarify the last sentence please. (not picking just learning) as I believed rcd's do not offer any overload protection.

Thanks mate

Matt

Hello Matty, Unphased made two comments that imply A) RCD's are required on all circuits under 17th edition and B) it is safer to just stick RCD's on rather then considering if they are needed.

So, given the advent of 17th edition and the total RCD protection (30mA) of all circuits, do we really need TN earths any longer? Should we be TT by default? Am I missing something?

With respect. This is not a mistake. It is far easier to install an RCD on a circuit and know it will comply than to spend ages justifying why there isn't.

These would both be providing some automatic disconnection of the supply under a fault condition, which is generally a good thing. But there are exceptions that a competent electrician should be aware of. As regulations 560.7.3 and 433.3.3 both discuss the need prevent automatic disconnection of the supply by omitting overload protection, it is reasonably safe to assume that in these situations automatic disconnection of the supply by earth leakage protection could also be more dangerous than "just sticking an RCD on and assuming it complies" I was not implying that RCD's provided any overload protection, which is why my comment included the word 'either'. and this is the reason I added my comments to the thread.

Just for the record there are situations that it is not appropriate to install overload protection either so a stick an RCD on everything can be more of a danger than a safety issue.

Doc H.

---------- Post Auto-Merged at 21:27 ---------- Previous post was made at 21:12 ----------

In reply to a PM from Unphased about this thread:

Originally Posted by unphasedI am sending you this message on a-one-to-one basis as it is not for general forum eyes.

I get the impression you are a senior or high ranking 'official' in the electical industry and clearly have a lot of experience and knowledge in this field. I find your forum posts, especially in response to mine, uneccesarily officious. You seem to diversify in to issues that are largely supplemetary to the source of a question and start mentioning "students" and "misleading" or "not clarified" about a post. This is a FORUM, doc, not a thesis on the whys and wherefores of a sunbject like I am sure you have written many in your past. I have intelligence as well as you and I don't feel the need to make condescending posts with underlying ridicule so I wonder why you do? Do youperhaps feel that you are too superior to the likes on me who just want a pleasant and nice discussion around a subject, with perhaps some naievety, or do you consider the need to impose your arrogance on the post. It matters not two hoots, one way or another, how you receive this private message as I have a life outside of forums and an intelligent enough mind to be able to get on without being treated by a school head reprimanding a scholboy because he hasn't, perhaps, risen to the delicate aret of factulal literature . There are som eproper thick idiots on the is forrum as well as incredibly intelligent and knowledgeable people. You seem to treat everyone as an idiot.

I really don't care if, as a result of theis email to you, I am banned or wahtever from this forum because it is not a very pleasnt place to be unless it is run YOUR way, is it!

Take care and good luck for the future.

Regards

unphased

I will not enter a private discussion about how the forum is run, all my comments will be public as it saves me wasting my time repeating information to others if a similar situation arises. This is a forum where many students do read posts and wrong impressions can easily be made from inaccurate statements. Whenever a member posts a statement that could be accidentally interpreted as an incorrect fact, such as your quote; "So, given the advent of 17th edition and the total RCD protection (30mA) of all circuits" then I, and many others, consider it poor judgment to ignore and not comment upon it. This allows debate to continue with a more balance view. We all want pleasant discussions around the various topics and there is little advantage to be gained by being unprofessional with basic facts and guidance.

Rather than using the PM system as an attack when someone holds a difference of opinion to you, It may be more constructive to just discuss your point and back up your opinions with industry standard guidance and regulations. I am just as open to being banned, given infractions or other warnings as you are, or any other member is and I prefer to let Admin, other Moderators and the forum as a whole vet my responses so as to avoid allegations of inappropriate comments made to members. As such this will be on open forum as I have nothing to hide. For the record, I consider some of your comments in the attached PM out of order and the PM system is not a means of getting around forum rules of polite and respectful posting. I apologies if others are also of the opinion as Unphased, that I treat everyone as an idiot. I leave others to draw their own conclusions and also note that the original post has been edited by unphased to slightly alter the context.

Doc H.

 

[Adrian]

I live in Cornwall and have been in the trade for about 25 years and I have never come across a lost neutral on tncs but I have come across loads of faulty rcd's or high ra on tt systems, so for me I would go for tn every time given a choice

 

[Manator]

I live in Cornwall and have been in the trade for about 25 years and I have never come across a lost neutral on tncs but I have come across loads of faulty rcd's or high ra on tt systems, so for me I would go for tn every time given a choice

I have, and very frequently. Only last month I did a test on a health centre where the lowest recorded reading for Ze was 0.52

4 connections down a loose connection was found, full climbing team took all day to find it. Best part of this story? the original EIC stated the high readings. Certificate issue date? 2003!

 

[OnOff]

Thinking out loud here which probably ISN'T a good idea after half a bottle of red, but..................if the multiple earthing of a true PME system is compromised (aka nicked by pikeys) then wouldn't it in effect "revert" to TN-C-S with a potentially higher Ze. Thinking further on then aren't you anyway supposed to install a rod on TN-C-S system (i.e. non PME). So.........would it not be belt and braces to RCD protect everything AND have a rod even if you have TN-C-S. As I say I've had a couple.............

 

[Manator]

Thinking out loud here which probably ISN'T a good idea after half a bottle of red, but..................if the multiple earthing of a true PME system is compromised (aka nicked by pikeys) then wouldn't it in effect "revert" to TN-C-S with a potentially higher Ze. Thinking further on then aren't you anyway supposed to install a rod on TN-C-S system (i.e. non PME). So.........would it not be belt and braces to RCD protect everything AND have a rod even if you have TN-C-S. As I say I've had a couple.............

I fail to see why members wish to apologise for bringing to the debate very reasonable argument.

 

[OnOff]

I fail to see why members wish to apologise for bringing to the debate very reasonable argument.

Result! Note to self: MUST DRINK MORE! Obviously brings a certain clarity!

 

[a3]

I am sending you this message on a-one-to-one basis as it is not for general forum eyes.

I get the impression you are a senior or high ranking 'official' in the electical industry and clearly have a lot of experience and knowledge in this field. I find your forum posts, especially in response to mine, uneccesarily officious. You seem to diversify in to issues that are largely supplemetary to the source of a question and start mentioning "students" and "misleading" or "not clarified" about a post. This is a FORUM, doc, not a thesis on the whys and wherefores of a sunbject like I am sure you have written many in your past. I have intelligence as well as you and I don't feel the need to make condescending posts with underlying ridicule so I wonder why you do? Do youperhaps feel that you are too superior to the likes on me who just want a pleasant and nice discussion around a subject, with perhaps some naievety, or do you consider the need to impose your arrogance on the post. It matters not two hoots, one way or another, how you receive this private message as I have a life outside of forums and an intelligent enough mind to be able to get on without being treated by a school head reprimanding a scholboy because he hasn't, perhaps, risen to the delicate aret of factulal literature . There are som eproper thick idiots on the is forrum as well as incredibly intelligent and knowledgeable people. You seem to treat everyone as an idiot.

I really don't care if, as a result of theis email to you, I am banned or wahtever from this forum because it is not a very pleasnt place to be unless it is run YOUR way, is it!

Take care and good luck for the future.

Regards

unphased

I would suggest you refrain from sending highly respected moderators private messages like that, the general tone and writing is not very pleasant

If you look at all of the Docs posts you will see that is his way of posting and he treats everyone the same - with respect

He is also a very knowledgeable contributor to this forum along with quite a few others

If you want to pick on a moderator then i suggest you PM Mr Smith as he will not be so nice and all your wishes will come true

If at any time you feel you are above (or below - or whatever springs to mind) this forum or its moderators or members and wish to leave then please feel free not to login again, if you really wish to be deleted then that can also be arranged, however with respect you will find this is a friendly place with good advice from good people if you want to try it out, this isnt the first time that you seem to have a problem here now is it

I will leave it at that and let you ponder

Have a good evening

 

[ParkySparky]

As previously said I have not posted many times on this forum I like to mostly browse as there's many usefull tips to be found.

Also with the knowledge on many people on here, including Doc H, many things can be cleared up.

So I personally think by him picking up on "small indescretions" as some people think they are, such as the one in the discussion above can be invaluable as young or inexperienced people could take whatever's written as gospel!

Nothing beats wisdom and experience, especially in our game as its ever evolving and there's always something knew to learn!

 

[Mr Smith]

mr phased

this is becoming a habit

your pm to mr hudson was

in mr smiths opinion

out of order

and it would be considered appropriate

by mr smith

for an apology to be forthcoming from yourself

mr hudson is a highly respected

and liked

member of this forum

mr phased

judging by the reputation received

does not currently fit either of these categories

mr smith would echo the comments of mr admin3

regarding mr phased and his attitude

for future reference

arguing a case with any moderator

through the pm system

will lead to the message being posted

with its reply

on the open forum

to allow the other members to make their own judgments

mr smith would suggest

that if mr phased wishes to pass as intelligent

he may wish to either

spell check his posts

or learn to spell properly

mr phased

mr smith suggests being aware that the current posting style

will not generate friends and positivity

perhaps mr phased needs to adjust his own attitude

before attempting to berate others

mr smith

 

[sb_89]

mr smith would suggest

that if mr phased wishes to pass as intelligent

he may wish to either

spell check his posts

or learn to spell properly

At the risk of incurring the wrath of Mr.Smith, that's a touch hypocritical. Not only is your grammar flawed, but I feel that's a personal attack, which is uncalled for and against forum rules

 

[Mr Smith]

mr 89

mr smith has an

addmittedly

peculiar posting style

however

we will leave that aside

to reply to your suggestion of

personal attack

on mr phased

on reading the pm referred

in post 11

one of the things mr smith became aware of

mr phased has a modicum of intelligence

given his usage of the english language

however

toward the latter part of the post from mr phased

the quality wanes somewhat

suggesting an outpouring of emotion

rather than fact

this is the essence mr smith was attempting to portray

and if mr 89 thought otherwise

then mr smith failed to convey his intent succinctly enough

and gives thanks to mr 89 for pointing this out

to return to the posting style

of mr smith

mr smith has always utilised his keyboard in this manner

in the land of forums

it is purely a quirk

in my professional life

it is the bane of my secretarys life

yet it is what it is

if the administrator views mr smiths post as a

personal attack

then mr smith will receive the same

as any other forum member

mr smith