single pole rcbo's in a tt system

[park1]

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, :shakehead

Rb is the resistance of the transformer earth.

Can we add resistance and reactance to find the impedance. So your telling me I can not take away a resistive element?

---------- Post Auto-Merged at 22:12 ---------- Previous post was made at 22:11 ----------

And I did say approximate

 

[steptoe]

Rb is the resistance of the transformer earth.Can we add resistance and reactance to find the impedance. So your telling me I can not take away a resistive element?

---------- Post Auto-Merged at 22:12 ---------- Previous post was made at 22:11 ----------

And I did say approximate

can you show/tell me where in the regs we work to that is referenced?

and the resistance of the Tx is constant for our calculations.

also, where in the world do you nget Zs - Ra = Rb[as you refer to] ?

its a fantasy in your mind.

the resistance of the Tx is absolutely noting to do with us, or our installations.

as for Rb, well,

:C

---------- Post Auto-Merged at 22:18 ---------- Previous post was made at 22:17 ----------

BTW, I'll approximate Rb [as you call it] for you, somewhere in the region of <21 ohms , now, when you get your maths to work out to that, come back to me.

 

[park1]

Steps come on, TT.

There's two electrodes in the fault path, Ra and Rb, you can't ignore Rb if your thinking of achieving low values.

Tx, constant, what you on about?

Zs on a TT or Ze if you prefer, what's the earth loop comprise of?

The line impedance will be low normally compared to Ra and Rb, do if you deduct Ra from Zs you will approx know Rb.

---------- Post Auto-Merged at 22:28 ---------- Previous post was made at 22:24 ----------

Ok Ra = 1 ohms. Rb 20 ohms. Line = .1 ohms

My maths say 21.1 ohms.

Oh dear all that hard work and Rb is 20 ohms.

Back to Rcd

 

[steptoe]

ok $park

you still havent told us where Rb has came from?

its not asking you to get me to the moon,

I simply want you to tell me where Rb has come from?

cos the earth at the Tx certainly isnt referred to Rb in any document I know of,

just let us in on your secret, thats all I ask.

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

 

[park1]

Oh and 21 ohms, not quite the case, it depends upon the Hv/Lb Earthing arrangement of the transformer, the key is EPR must not exceed 430 volts.

---------- Post Auto-Merged at 22:35 ---------- Previous post was made at 22:33 ----------

Combined earth systems should not exceed 1 ohm, this has been excluded from ESQCR but remains in BS 7430.

---------- Post Auto-Merged at 22:38 ---------- Previous post was made at 22:35 ----------

Oh, by the way it is in BS 7671, page 39 BGB

---------- Post Auto-Merged at 22:41 ---------- Previous post was made at 22:38 ----------

There's no such thing as a constant, just what the dno achieve dependent upon what Earthing arrangement.

 

[Jono Pashley]

Zs-Ra would not give you your "Rb" FACT.

 

[park1]

Rb alias Source Earth

---------- Post Auto-Merged at 22:47 ---------- Previous post was made at 22:42 ----------

Zs-Ra would not give you your "Rb" FACT.

Approximatly it will, if Rb is extremely low then it Will be less accurate agreed.

So Zs at the intake = 21.1 ohms. Ra measured 1 ohm. So Rb approx 20 ohms.

Where's the problem, care to explain.

 

[OnOff]

Can I play? WTF is Ra? Isn't it RA?

As to where is it referenced then try "Symbols Used In The Regulations" in 7671

As to "what it is", it says this:

"Resistance of the earthing arrangement of the low voltage system neutral, for low voltage systems in which the earthing arrangements of the transformer substation and of the low voltage system neutral are electrically independent"

(I actually read that last night and didn't understand it then so it's Cabernet Shiraz time! )

:Y

 

[Jono Pashley]

What a load of rubbish if you take a "Zs" at a DB so MEBs connected you will have additional paths besides the rod you installed so your Ra suddenly means squat

 

[park1]

What a load of rubbish if you take a "Zs" at a DB so MEBs connected you will have additional paths besides the rod you installed so your Ra suddenly means squat

Get with it jono.

We disconnect the Earthing conductor and test between phase, so in essence Ze, BS 7671 states where Ra is not known it can be replaced with Zs.

So, Ze Zs, I don't mind either, I prefer Ze. So as I said, please explain, only the external loop is under test.

 

[kme]

I`ve had waaaaaaaaaaaaaaaaaaay too much of Mr. Artois` best product to form an opinion on this right now..........................

Have to re-assess tomorrow, methinks

 

[steptoe]

Get with it jono.We disconnect the Earthing conductor and test between phase, so in essence Ze, BS 7671 states where Ra is not known it can be replaced with Zs.

So, Ze Zs, I don't mind either, I prefer Ze. So as I said, please explain, only the external loop is under test.

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

Zs or Ze ?

it makes a massive difference.

the other thing is this,

for me, I dont deal in approximations, I deal in actual factual measurements, someones life may depend on it being correct,

or, I could guess and do a drive by PIR I suppose, like the rest of the cowboys,

either deal in facts and measurements, or just guess everything,

what one is it you do $park ?

BTW, I think someone let your horse loose.!

 

[OnOff]

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

it makes a massive difference.

the other thing is this,

for me, I dont deal in approximations, I deal in actual factual measurements, someones life may depend on it being correct,

or, I could guess and do a drive by PIR I suppose, like the rest of the cowboys,

either deal in facts and measurements, or just guess everything,

what one is it you do $park ?

BTW, I think someone let your horse loose.!

Pretty sure the regs say that if a EICR can't be done as a drive by then you can use Google Earth as an alternative................ :coat

 

[Jono Pashley]

I`ve had waaaaaaaaaaaaaaaaaaay too much of Mr. Artois` best product to form an opinion on this right now..........................Have to re-assess tomorrow, methinks

Speaking of which i have some in the van for you

 

[park1]

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

it makes a massive difference.

the other thing is this,

for me, I dont deal in approximations, I deal in actual factual measurements, someones life may depend on it being correct,

or, I could guess and do a drive by PIR I suppose, like the rest of the cowboys,

either deal in facts and measurements, or just guess everything,

what one is it you do $park ?

BTW, I think someone let your horse loose.!

I can see several horses running around on this thread, "Electrician's" and no knowledge of TT Earth Fault Loop's, BASIC STUFF!!

Okay, if we want to use MCB's on a TT we CAN NOT solely rely on the Ra measurement, reason, because Rb maybe as high as 20 ohm's, can we understand that or is that a little difficult. So we need to measure Ra and Rb, the Reactance of the TX and the impedance of the Phase. So to use MCB's on a TT we need to ensure that the source earth (Rb) is low and give some headroom for the consumers Ra.

If we are using RCD's we do not need to Find Ra, we can use a earth loop impedance tester as per 411.5, the measured Zs can be used as th Ra.

Why Zs when the earthing conductor is disconnected, well technically the earth electrode Ra is part of the consumers installation so not external hence Zs in 411.5, personally i don't mind it being called Ze but i think you will find BS7671 GN3 etc only refer to Zs.

As for subtracting, its a guide steptoe, if the DNO are unable for some reason to give you the resistance of Rb you could drive in a quick rod , measure Ra then do a quick loop impedance. If we then subtract Ra from Zs it tells us roughly what Rb is going to be, not much use agreed, if you were installing MCB's you need to measure Rb and Ra accurately, I was just showing you the existence of Rb and how you could test and see it existence, certainly where there is no Hv/Lv bond at the TX.

So steptoe, Is my my horse or your horse on the loose. Feel free to prove me incorrect.

 

[Sidewinder]

And how do we control the value of Rb?

Having checked the term, as I said, it was late & I was tired, with the regs in the van as always.

We don't control Rb it's not part of our install.

It is governed by the DNO & their install.

---------- Post Auto-Merged at 11:01 ---------- Previous post was made at 10:40 ----------

lmfao, that made me chuckle.Subtract Ra from Zs to approximate Rb

You "shouldn't" really add & subtract R & Z values, as they are not "really" the same thing are they, however, I'll go with an approximation.

I believe your Zs should be at the origin to guesstimate Rb rather than a circuit

Also, where Rb is noted in the regs is the new section wrt EMI rather than efli.

 

[apprentice87]

Hi all,

Now look, i am only a newbie right, but here is how i see it..... Rb is as defined by Onoff;

"Resistance of the earthing arrangement of the low voltage system neutral, for low voltage systems in which the earthing arrangements of the transformer substation and of the low voltage system neutral are electrically independent"

What this means, i have really no idea, but i presume that it is the equivalent to Ra, for the rod at "their" end, you know, the one that must not exceed 21 ohms.

Now then, if we test an earth rod properly, we measure Ra, as is correctly, so often pointed out by Steps.

As a matter of interest, i know if does say in the regs that you can as an ALTERNATIVE just measure Zs instead, but does this not infer that Steps "proper" Ra method is the preferred one??? I think it does...

You will notice that i referred to Zs and NOT Ze... Why is this?? Because Ra includes the resistance of the cpc connecting the rod to the exposed conductive part in question, so Zs it is.....

Anyway, I digress...

So, as Sidewinder points out, Rb has nothing to do with us...

Firstly, we are only concerned with the impedance of the earth fault loop as a whole, [we only measure Ra to make sure that we have a reliable connection to the mud the thing is bashed into] and secondly, i do not think we can infer anything from Rb at all, as we do not know the impedance of the ground between our rod and theirs, and thirdly; How you going to measure Rb in the first place??????? Climb over the fence into the substation, disconnect their earthing conductor and get your set of test rods and your MFT out????? I think not..

Worrying about Rb is much like worrying about how far away the sun is, or the wavelength of light, who cares?? Nothing you can do about it, and so long as it gets light tomorrow, that will do me

Lot of fuss about nothing......

john...

 

[OnOff]

Hi all,Now look, i am only a newbie right, but here is how i see it..... Rb is as defined by Onoff;

"Resistance of the earthing arrangement of the low voltage system neutral, for low voltage systems in which the earthing arrangements of the transformer substation and of the low voltage system neutral are electrically independent"

What this means, i have really no idea, but i presume that it is the equivalent to Ra, for the rod at "their" end, you know, the one that must not exceed 21 ohms.

Now then, if we test an earth rod properly, we measure Ra, as is correctly, so often pointed out by Steps.

As a matter of interest, i know if does say in the regs that you can as an ALTERNATIVE just measure Zs instead, but does this not infer that Steps "proper" Ra method is the preferred one??? I think it does...

You will notice that i referred to Zs and NOT Ze... Why is this?? Because Ra includes the resistance of the cpc connecting the rod to the exposed conductive part in question, so Zs it is.....

Anyway, I digress...

So, as Sidewinder points out, Rb has nothing to do with us...

Firstly, we are only concerned with the impedance of the earth fault loop as a whole, [we only measure Ra to make sure that we have a reliable connection to the mud the thing is bashed into] and secondly, i do not think we can infer anything from Rb at all, as we do not know the impedance of the ground between our rod and theirs, and thirdly; How you going to measure Rb in the first place??????? Climb over the fence into the substation, disconnect their earthing conductor and get your set of test rods and your MFT out????? I think not..

Worrying about Rb is much like worrying about how far away the sun is, or the wavelength of light, who cares?? Nothing you can do about it, and so long as it gets light tomorrow, that will do me

Lot of fuss about nothing......

john...

............RB

 

[park1]

Having checked the term, as I said, it was late & I was tired, with the regs in the van as always.We don't control Rb it's not part of our install.

It is governed by the DNO & their install.

I never said it wasn't, just stating that your Ra of 1 ohm may be dramatically increased by Rb, what the difficulty in that?

---------- Post Auto-Merged at 11:01 ---------- Previous post was made at 10:40 ----------

You "shouldn't" really add & subtract R & Z values, as they are not "really" the same thing are they, however, I'll go with an approximation.

I believe your Zs should be at the origin to guesstimate Rb rather than a circuit

Also, where Rb is noted in the regs is the new section wrt EMI rather than efli.

Well you SHOULD and need to add R and X values(not Z, that's the resultant impedance), they are not the same thing but none the less can be added, subtracted, divided or multiplied, just not in the conventional way.

Rb has been around for years, it's the solid earth of the Tx which is a requirement where T is the first letter of the earthing arrangement.

Apprentice, you worry about Rb if you want a system which utilises MCB's for example, or where its a Private Tx, there are requirement as to the impedance of Rb.

---------- Post Auto-Merged at 14:09 ---------- Previous post was made at 13:32 ----------

Paul you should know better, you teach the 2391, if you draw the fault loop for a TTand don't include Rb you will loose marks.

To the other's, this is fundamental stuff, the fact you aren't aware of Rb is a worry. It's not rocket science, what's the first T stand for, simple stuff, basics.

There also seems to be a lack of knowledge for the math.

I suppose this is due to the dumbing down of our industry.

Most of you work for yourselfs, you should know the theory, you have to design, it's quite worrying.

 

[Sidewinder]

Well you SHOULD and need to add R and X values(not Z, that's the resultant impedance), they are not the same thing but none the less can be added, subtracted, divided or multiplied, just not in the conventional way.Rb has been around for years, it's the solid earth of the Tx which is a requirement where T is the first letter of the earthing arrangement.

Apprentice, you worry about Rb if you want a system which utilises MCB's for example, or where its a Private Tx, there are requirement as to the impedance of Rb.

---------- Post Auto-Merged at 14:09 ---------- Previous post was made at 13:32 ----------

Paul you should know better, you teach the 2391, if you draw the fault loop for a TTand don't include Rb you will loose marks.

To the other's, this is fundamental stuff, the fact you aren't aware of Rb is a worry. It's not rocket science, what's the first T stand for, simple stuff, basics.

There also seems to be a lack of knowledge for the math.

I suppose this is due to the dumbing down of our industry.

Most of you work for yourselfs, you should know the theory, you have to design, it's quite worrying.

If, you recall I asked for a reminder, it was late, & cannot recall all aspects of the regs & all the other stuff I do at all times!

With regard to the teaching, yes I do, and, I do teach to put in the source earth rod, but, the students have enough to remember without them remembering the abbreviations!

Yes they should, and yes these should be on the drawing, but, not many people have a photographic memory!

As I don't teach 2391 "every week" when I do have a session coming up I prep for it, by running through the teaching materials and the relevant areas in the regs, I don't have a photographic memory either, and have a lot on my plate.

For all domestic sparks, there is nothing they can do about Rb anyway, so where is little to be concerned with, unless you are looking at a PNB install perhaps, or a single domestic TT/TN from a dedicated transformer, but, again, nothing can be done about it anyway, it is, what it is.

As far das Onoff picking up App for the Rb/RB thing, it is difficult, well I don't think I can on this device add subscripts, it "should" be capital R, with subscript capital A for the earth rod resistance, however, most understand the Ra term.

I did not say that Rb had not been around, I said that as far as I knew it is only currently explicitly mentioned in chapter 44, apart from the definitions.

There may, be more uses, but, In the quick check I did, I could not find them.

If you have a private Tx then,, this is a situation where you are worried about Rb, but would a "run of the mill" electrician be involved in the design, installation, & commissioning of an HV/LV transformer alone? I doubt it tbh.

In a normal situation, would you really have sufficient info to undertake the required calculations, considering that if you are using say a conventional MFT to take the readings, they are measured in different ways, one is a d.c. resistance, the other is actually an impedance measurement?

I never said that you should not add/subtract, resistance and reactance, I stated impedance, and if you read the post, you said approximate, and, I agreed that whilst wrong, it would give you an approximation.

Gonna try to find a way of handling sub/superscripts on this pad,me back later!

Got an hour to kill here.

---------- Post Auto-Merged at 14:53 ---------- Previous post was made at 14:41 ----------

It works!

I could not get the advanced reply to function before, now it does, so:

RA & RB

Woo hoo!