single pole rcbo's in a tt system

[apprentice87]

Hi all,

What Steps said about bonding etc, is EXACTLY 1000% correct..... If your TT fault current can travel up your neighbours rod/extraneous parts, and hence to neutral/star point, then it will work the other way too..

Say your TNCS neighbour has a lost neutral on his supply, so all his metalwork rises in potential, up towards full mains voltage is possible, [though unlikely] but you will in any case still see a dangerous rise, anyway, SO WILL YOURS NOW, thanks to his [your neighbours that is!!] extraneous bits, being either connected directly to yours, or them being near to your TT rod....

As Steps said, TNCS is a barmy system, barmy enough that the DNO's are getting scared to use it now, and are increasingly, [or so i have been told by a cable jointer] telling you to provide your own earth..

john...

---------- Post Auto-Merged at 16:19 ---------- Previous post was made at 16:05 ----------

Think of it like this;

What do you suppose would happen if you took your car for an MOT, only, you had first thrown away, or otherwise disabled, the dual circuit braking system on the grounds that; "the pipes are in parallel with the other ones, so i decided, [in the interests of economy] to throw them away.."

"It is ok, because if the brakes fail, i will be able to slow down by leaning on the car in front"

Much the same as TNCS is ok, because "well, if we lose the neutral, they have got Eq Bonding, so that makes it ok"

john...

 

[batty]

Things were fine with TNCS but as these days things have changed with rising copper prices utility companies have the problem of theft of the PME cabling. Which as APP87 is a big concern for DNO.

 

[Steve3948]

and that my friend is why bonding etc has to be so large on a TNCS/PME system, you can pull faults in from neighbouring properties,and who said TNCS/PME was a good idea?

Well I use 2.5mm

 

[steptoe]

Well I use 2.5mm

 

[Steve3948]

and how would they know about it if no-one reports it then?

Well I assume because more users would if they lost their neutral or had no supply, but 1 rod WGAS.

 

[steptoe]

still not any reason not to have your electrical installation tested at reasonable intervals, nobody has a problem getting their gas checked yearly.

anyway, it may still work on a PME if you dropped the neutral. :|

 

[springcrocus]

Well I assume because more users would if they lost their neutral or had no supply, but 1 rod WGAS.

If the bonding was good, they might not notice any loss of supply with a dropped neutral.

 

[park1]

Hi All,$park, when you talk about Rb, we all know what it is, the resistance of the rod at the transformer end, we all know what it should be, 21 ohms max, but we do not all know the "shorthand" term for it, as, like Sidewinder said, it is of no interest WHATSOEVER to a normal electrician.

Ermmm, which thread have you been reading? Several seem not to know!

So you might use MCB's in an install, so what, if it is a TT install you will HAVE [normally] to use an RCCB in addition to give earth fault protection, so as long as your loop impedance is RELIABLY low enough, that the operating current of the RCCB does not cause touch voltages to go about 50V, who cares about anything else????

RCD's don't limit touch voltage.

If the DNO do not care about maintaining the TNS earth, what sort of response you think you going to get if you phone up and say "good afternoon, i have an Ra of 5 ohms, but i cannot get Zs below 20 ohms, so there must be something wrong with the rod at your end????"

I never said it was a problem, just something to be aware of.Just basic theory.

john...

Regards

---------- Post Auto-Merged at 08:57 ---------- Previous post was made at 08:19 ----------

It seems like $park is changing his "arguments" as time goes on, it seems that the point currently being made is different to that previous?Well?...

I'm not arguing, where have i changed my mind?

---------- Post Auto-Merged at 09:04 ---------- Previous post was made at 08:57 ----------

Hi Steps,It is my understanding of all this, [rightly or wrongly!!] that Rb, is EXACTLY the same as "our" Ra, but at the DNO end, so the fault path in a TT system is down our earthing conductor, into the ground via our, hopefully, much less than 200 ohms Ra rod, through the ground, up their 21 ohms Rb [stated maximum] rod, and then to the star point..

Which is all i have stated.

Now, although we might WANT an Ra of less than 200 ohms for "our" rod, their rod MUST [although i do not think it is actually a LEGAL requirement] have an Rb of less than 21 ohms.

Well there is a statutory requirement, it is the EPR which governs the resistance. Do you really think it would be acceptable to have a 11KV fault appearing on the LV side?

So, in other words; the generally assumed maximum earth fault loop impedance [although it is most certainly not a legal requirement] for the various earthing systems [correctly] measured at the cutout position or origin of our installation is going to be;

TNS 0.8 ohms [in other words Ze = 0.8 Ohms]

TNCS 0.35 ohms [in other words Ze = 0.35 Ohms]

TT 21 ohms [in other words Zs = 21 Ohms] [it would be correct i suppose, if maybe pedantic, to say Zs and NOT Ze, as it includes our rod which is obviously NOT external to our installation, cos we own it..]

As i have already stated.

Their rod, as it is connected to the star point, also, as has been stated, ties the neutral down.

Again as i stated.

Once again though, as you rightly say, hardly relevant to any of us.... It is a DNO problem, unless as Sidewinder says, we are in the business of installing private substations!!!!!

Its just basic theory, fundamentals, something electricians should be aware of.

john...

Regards

 

[Steve3948]

still not any reason not to have your electrical installation tested at reasonable intervals, nobody has a problem getting their gas checked yearly.anyway, it may still work on a PME if you dropped the neutral. :|

That's my point, the public don't see getting their installations checked at reasonable intervals as a priority, it's only when something goes wrong, or they sell the house.

Every time I go into a house with Rcd's I ask them when was the last time they tested the Rcd, it's always a shrug of the shoulders, so never.

If they don't check that I'm sure they don't check their rod, if they even know they had one.

 

[park1]

Steptoe

why cant you just explain to me what Rb is?Ive never heard of it,

and you cannot, CANNOT, subtract a resistance from an impedance, they are 2 different things,

Incorrect, you CAN subtract a Resistance from an impedance. You need to get your math sorted!!

BTW, resistance at the Tx is a constant =<21

Really, their earthing systems always achieve a constant 21 ohms,best hope on combined systems there is no HV/LV faults, LMFAO.

right, so what is it then you are measuring/subtracting?Zs or Ze ?

it makes a massive difference.

As i said, technically its Zs, but call it Ze if you like.

But the neutral isnt the earth in a TT system, is it,? That is the whole ethos of TT.Now who's horse is on the loose?

I never said it was? I said what do you think the first letter T denotes, simple.

no, why would we be using a neutral resistance measurement to work out Ze, Zs, Ra or any of the other earthing parameters on a TT system?I really dont understand why the neutral has any bearing on the earth loop of a TT system,

hopefully you will be able to explain this to me $park,

hopefully, but I have serious concerns of your ability to do so, or even mr Tesla's original namesake being able to do so,

but, maybe I have a lot more to learn than I thought

Well you need to explain where and when i stated this, i don't believe i have?

the earth at the Tx must not be greater than 21

Again, incorrect, you would kill people on a combined system at 21 ohms.

But RB is only of relevance on a TN system, it is a neutral resistance, on a TT we are only concerned with the earth resistance, or impedance, whatever one floats you sailing device.

What on earth are you on about. Rb is the solid earth of the Tx, NOT a neutral resistance. It creates the Star Point, as i said it ties the neural down. in a TT Earth fault current flows through Rb, its part of the Earth fault loop so is relevant To a TT NOT TN in that respect, its relevant to all in other respects.

So if we have a TT Three phase, Rb has no relevance, so lets do away with it, can you explain what will happen Steptoe.

For someone who purports to be TT specialist you know very little!!

 

[steptoe]

1 I never purported to be a TT specialist

2 RB = resistance of the earthing arrangement of the low voltage system neutral.

3 RE = resistance of the earthing arrangement of the transformer substation

so, kindly explain again to me why RB is of concern in a TT system.

 

[park1]

1 I never purported to be a TT specialistOkay, a misconception, i apologise.

2 RB = resistance of the earthing arrangement of the low voltage system neutral.

3 RE = resistance of the earthing arrangement of the transformer substation

And your point is?

so, kindly explain again to me why RB is of concern in a TT system.

I think the above post draws a pretty clear picture as to the importance of Rb im not going to add further to this thread.

Although practically to an electrician Rb may seem irrelevant, its of great importance, as practicing electricians i would expect you to know the fundamentals, its pretty basic stuff.

If you lost Rb you could have Line voltages up 400 V +/-, If the resistance of Rb is to high, say on a combined, a Hv fault could place several thousand volt on the Lv side.

So knowing the theory is important, don't knock just because of your ignorance.

 

[apprentice87]

Hi $park,

Where i said;

"So you might use MCB's in an install, so what, if it is a TT install you will HAVE [normally] to use an RCCB in addition to give earth fault protection, so as long as your loop impedance is RELIABLY low enough, that the operating current of the RCCB does not cause touch voltages to go about 50V, who cares about anything else????"

You Replied that;

"RCD's don't limit touch voltage"

I think i must have misunderstood what you mean, as I never said they did;

What i said was, once again; "[ramble ramble] so as long as your loop impedance is RELIABLY low enough, that the operating current of the RCCB does not cause touch voltages to go about 50V"

This is why, although a 30mA rcd would operate ok with a loop impedance of 230V/.030A = 7666 ohms, because in practice you have to ensure that touch voltage is less than 50V, the ACTUAL loop impedance must be less than 1667 ohms, because 50V/0.30A = 1666.666 ohms..

RCD's MCB's fuse's it is all the same, what limits touch voltage, is the impedance of the "earth" half of your fault loop path. So, you have a fault current of 342 Amps say, and the impedance of the "earth" half of your fault loop is 0.4 ohms, you will have a touch voltage of 342 x 0.4 = 136.8 volts.. Oooerrr!!!!

As i said, i am only a newbie, so i have no desire to get into any technical arguments with people that have VASTLY more knowledge than me, but i do know a little bit!! [i hope!!!]

john...

 

[steptoe]

I still dont know how RB affects the RA or Ze of a TT system, can you explain this to me please, Im obviously a bit stupid.

Although practically to an electrician Rb may seem irrelevant, its of great importance, as practicing electricians i would expect you to know the fundamentals, its pretty basic stuff.If you lost Rb you could have Line voltages up 400 V +/-, If the resistance of Rb is to high, say on a combined, a Hv fault could place several thousand volt on the Lv side.

So knowing the theory is important, don't knock just because of your ignorance.

 

[eamonn1983]

Got my own company Steptoe

 

[steptoe]

Got my own company Steptoe

Good man, you'll be able to afford to buy me a pint next week then!

 

[OnOff]

Fuelled by Nottage Hill I found the link below: Page 21's definition of RB makes interesting reading as to what RB is................

http://www.international.bticino.com/en_US/browser/attachments/bin/techguide/C_55_EditorialContent_67_TechGuide_Properties_TG_AList_GT_AItem_9_GT_ABin.pdf

And TBH the only thing you can rely on, and that ONLY at the time of test is your measurement at the rod end. You have NO control over the mass of earth or their rod (I've steered clear of calling in RB in light of this find).

Then......re-reading the BRB (sorry, not gone green yet) and it says "the resistance of the earthing ARRANGEMENT of the low voltage system neutral". I would say therefore that "arrangement" means all of 'em - theirs, yours etc. So, sorry BUT RB AIN'T JUST THEIR ROD!

Pick the bones out of that!