single pole rcbo's in a tt system

[apprentice87]

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.

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????

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????"

john...

 

[steptoe]

my apologises, I didnt realise it had now been added to the regs,

but, why and how does Rb concern us, and why worry about the resistance of the neutral,?

going by the way Im reading the definition in the BGB Re[E] is what you need to be concerned with on a TT system? or am I reading how they define stuff all wrong? :|

 

[apprentice87]

Hi Steps,

Presumably, as the neutral now doubles as an earth....

[At least i think that is what they mean]

Still, as you said, it has NO relevance to any of us anyway.

john..

 

[steptoe]

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?

 

[apprentice87]

Hi Steps, that is just me posting when i am half asleep!! Forgot it was TT we was on about..

john...

 

[park1]

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?

Yours, big time.

Oh dear steps, what do you think ties the neutral down. Come on, your trying to wind me up surly?

 

[steptoe]

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, :|

 

[OnOff]

Right, I can't even blame these comments on the drink as I've only had the one (that's about to change) but isn't considering RB is PRACTICAL terms a bit of a waste of time on a TT system? I'm not saying it's irrelevant but restricted to an ideal world. Shoot me down by all means but here goes: In this ideal world then the fault path (quite literally "the earth") WOULD be of a low enough resistance to go back to the star point/neutral at the source. You strive to get RA as low as possible on a TT but it's potentially always going to be a variable i.e. NOT guaranteed (stable?). TBH if you could guarantee stability and thus quick disconnection times then you could think about ditching RCDs (take cover!!!). So........we try and get our RA down as low as we can, get it to a cracking <1ohm with our mutiple rods etc...........then the regs dictate we'd best whack an RCD in just in case RA goes up! Hope that makes sense. Back to then "why worry about RB" - cos TBH you can't guarantee RA only get it as low as you can and periodically check it. Think we need to accept that RB "exists" and leave it at that without worrying about it. :Y

 

[steptoe]

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.

---------- Post Auto-Merged at 23:32 ---------- Previous post was made at 23:30 ----------

Even my phone has the intelligence to change RB to TN on autospell FFS.

 

[OnOff]

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.---------- Post Auto-Merged at 23:32 ---------- Previous post was made at 23:30 ----------

Even my phone has the intelligence to change RB to TN on autospell FFS.

Just re-read my post, should have perhaps posed that RB is irrelevant in practical terms on a TT (though it still forms part of the fault path......).

 

[Sidewinder]

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?...

 

[steptoe]

Its all this copy and paste where someone else has made a.wrong statement, now has to wait for a reply .

 

[apprentice87]

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..

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.

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..]

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

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!!!!!

john...

 

[unphased]

That is an excellent post, John. Very well put indeed. You get a scoob for that.

 

[apprentice87]

Thank you very much!!

john..

 

[Steve3948]

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..

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.

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..]

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

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!!!!!

john...

John, I don't see TT @ 21 ohms as being correct, if the recommended Ra is 200 ohms, could you explain how Zs will ever be 21 ohms.

 

[steptoe]

the earth at the Tx must not be greater than 21 ohms

 

[binky]

If my rod reads less than 200 ohms, and r1+r2 numbers for the circuits concerned are compatible with non TT systems showing that I've made a good job of the circuits, Then i really couldn't give a monkeys about Rb even if it interesting to know that it exists, and certainly wouldn't worry about it before deciding whether to use MCBs RCDs or any of the other avaiable prtoection devices. ( I'm hoping no-one else has already said this cos I'm wading in late as usual)

 

[apprentice87]

Hi Steve,

Steps summed it up in one!!!

The figure for Ra of 200 ohms for "our" earth rods is a recommended MAXIMUM, obviously most are a LOT lower..

Meanwhile, the DNO have to get the rod at their end to a stated maximum of only 21 ohms.

What they actually achieve in practice, i have no idea, generally a lot less i would think..

Anyway, our Zs [in a TT system] is [in theory] made up of the total combined impedance of the phase conductor up to the point where we are doing our measuring and the impedance of the circuit cpc, the earthing conductor, the earth rod itself, the ground in between us and "their" rod, and then the impedance of their rod and their earthing conductor feeding the fault current back to the star point.

In practice i expect the fault current returns to the Tx along MANY paths, not neccessarily up their rod either..

For all i know, an easier return path would be down our bonding conductors to the water and gas pipes, along the pipes, up to next doors MET [also bonded to the same pipes!] then to their TNS or TNCS earthing terminal and back that way!!!!

Say, you are on TT, but next door are on PME. In theory at least, where their service joins the LV main, there should be an earth rod about 6 foot long...

Presumably then, your TT fault current only has to go as far as their rod, and then directly back along the neutral..!!

john...

 

[steptoe]

Hi Steve,Steps summed it up in one!!!

The figure for Ra of 200 ohms for "our" earth rods is a recommended MAXIMUM, obviously most are a LOT lower..

Meanwhile, the DNO have to get the rod at their end to a stated maximum of only 21 ohms.

What they actually achieve in practice, i have no idea, generally a lot less i would think..

Anyway, our Zs [in a TT system] is [in theory] made up of the total combined impedance of the phase conductor up to the point where we are doing our measuring and the impedance of the circuit cpc, the earthing conductor, the earth rod itself, the ground in between us and "their" rod, and then the impedance of their rod and their earthing conductor feeding the fault current back to the star point.

In practice i expect the fault current returns to the Tx along MANY paths, not neccessarily up their rod either..

For all i know, an easier return path would be down our bonding conductors to the water and gas pipes, along the pipes, up to next doors MET [also bonded to the same pipes!] then to their TNS or TNCS earthing terminal and back that way!!!!

Say, you are on TT, but next door are on PME. In theory at least, where their service joins the LV main, there should be an earth rod about 6 foot long...

Presumably then, your TT fault current only has to go as far as their rod, and then directly back along the neutral..!!

john...

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?