Bonding to gas and water block of flats

[MSH1601]

I'll go over it again for you so you understand what happened because from what you are posting, you've obviously missed something or assumed something.

A large Victorian town house has one supply.

The house had been converted into flats.

I took a Ze in the flat we were working in (by saying I took a Ze, you are welcome to assume I know how to do this).

I had a reading (can't remember the reading but it was reasonable for TT(yes I know what TT is)). I therefore wanted to see the earth spike.

It was disconnected.

Boss asked rental agent for access to look at Supply.

We found supply with 2 earth blocks next to it.

The flats were connected to them.

They weren't connected to the earth spike.

Reconnected spike.

Boss called DNO to arrange PME.

Therefore... When I saw this post I asked what I asked.

 

[steptoe]

Thats not a Ze, a Ze is at the source with the earthing conductor disconnected, you done a Zdb, I know thats not a proper term, but it seems to be an accepted one.

 

[MSH1601]

My apologies. I stand corrected.

 

[MSH1601]

Ze? With muli flats bonding incoming supplies.. Does that mean your gonna need to knock on all doors to disconnect everyone's bonding to get a Ze?

If you were taking a measurement of Ze for the whole building you would disconnect the 'Earthing Conductor' from the MET, at the origin of the installation - i.e. the main board where the supply enters the building - and measure in the normal way.

If you were working for an individual tenant and treating one of the flats as a seperate 'installation' in it's own right, then you would disconnect that particular flat's incomming earth from it's EMT (sub-MET) and measure Ze at the flat's CU, in the normal way.........external to the installation(flat).

If you were working for an individual tenant and NOT treating the flat as a seperate installation, but as a sub off the main board, then Ze would be measured at the 'origin' (main board), and the measurement at the EMT in the flat would be Zs of the sub-main..........with earthing and bonding connected.

In which of the above three circumstances do you envisage knocking at every flat to disconnect their bonding??

Hi ADS,

Just to let you know that in the second circumstance. The reading is a Zdb not a Ze. I've been corrected.

 

[ADS]

Hi ADS,Just to let you know that in the second circumstance. The reading is a Zdb not a Ze. I've been corrected.

No it's not.

In the second circumstance we are treating the electrics inside the flat as a seperate installation in it's own right - therefore, the earth loop external to this installation will, by definition, be Ze.

There isn't an abbreviation called Zdb anyway, (well not in my green book), so, it's either Zs (of the sub-main circuit feeding the DB), or it's Ze (external to the installation).

There is a Z, Zl, Ze, Zk, Zm, Zs, Zx, Zls.................but, NO Zdb!

 

[steptoe]

right ADS, I did make it clear in my post that Zdb wasnt an actual certified reading, but was IMHO accepted by most electricians, Im not including 5WW in that statement BTW,

he has both an MET and multiple EMTs, so what do you propose to do?

 

[apprentice87]

Hi all,

Now, i am not getting involved in arguments with people with a LOT more knowledge than I. So, i will merely confine myself to making an observation [or two!!]

First off, every installation has only ONE origin, secondly, every installation has only ONE MET..

Where you have an installation split into several "components" shall we say, [you know what i mean!!] fed by a series of submains, this does not change things, you still have the one MET, you still have the one earthing conductor. Everything after the MET is a CPC.

All the other "MET's" are in fact "BEMT's"

Now look at this from the point of view of the DNO.... You go to a particular flat, and measure Zs or Zdb if you like and say it is 1.2 ohms.

It is no good then, phoning the DNO and saying the Ze for my flat is too high.

They will come along, test Ze at the origin [their cutout] pronounce it ok, and tell you that the rest of the wiring belongs to you [not them] and so it is part of YOUR installation not theirs. They will just say their Ze is ok, and suggest to you that if Zs in your flat is too high, that you get yourself a lower impedance submain, or TT the lot....

john

 

[Sharpend]

I like the idea of TT in a flat, I have visions of all the green and yellow snaking down the external walls, especially if it's 20 floors high, what a sight.

 

[apprentice87]

Blocks of flats near here have all the gas installation pipework, in copper, run on all the external walls!!! always paint them green and yellow and use them!! [Well, perhaps not!!!!]

john....

 

[OnOff]

I like the idea of TT in a flat, I have visions of all the green and yellow snaking down the external walls, especially if it's 20 floors high, what a sight.

How about putting the rod in the roof garden to save on cable? :coat

 

[Sharpend]

Was thinking more window box.

 

[ADS]

Hi all,Now, i am not getting involved in arguments with people with a LOT more knowledge than I. So, i will merely confine myself to making an observation [or two!!]

First off, every installation has only ONE origin, secondly, every installation has only ONE MET..

Where you have an installation split into several "components" shall we say, [you know what i mean!!] fed by a series of submains, this does not change things, you still have the one MET, you still have the one earthing conductor. Everything after the MET is a CPC.

All the other "MET's" are in fact "BEMT's"

Now look at this from the point of view of the DNO.... You go to a particular flat, and measure Zs or Zdb if you like and say it is 1.2 ohms.

It is no good then, phoning the DNO and saying the Ze for my flat is too high.

They will come along, test Ze at the origin [their cutout] pronounce it ok, and tell you that the rest of the wiring belongs to you [not them] and so it is part of YOUR installation not theirs. They will just say their Ze is ok, and suggest to you that if Zs in your flat is too high, that you get yourself a lower impedance submain, or TT the lot....

john

Very good, Apprentice87, all that remains for you to do now is define 'an installation'.

If you're wondering why I'm asking you to do this, it's because the definition of 'Ze' is the 'earth fault loop impedance' external to the installation - nothing to do with DNOs or anything else.

An installation supplied by a generator still has an 'origin', a Ze, a MET and a 'earthing conductor...... but no DNO.

And, as you correctly state, an installation only has one MET and one 'Earthing Conductor' ...........so if you can define 'installation' for us, happy days.

Or, perhaps it would be easier for you to tell me why the flat in the Ops scenario can't be treated as a seperate installation.

 

[steptoe]

its a sub-main supplied via an OCPD from a remote location that the tenant/landlord is responsible for,

their is no direct connection to an earthing system from the flat, it goes to a remote MET.

 

[ADS]

its a sub-main supplied via an OCPD from a remote location that the tenant/landlord is responsible for,their is no direct connection to an earthing system from the flat, it goes to a remote MET.

And your point is??

 

[steptoe]

And your point is??

:shakehead

if you are unable or unwilling to grasp the concept of a seperate installation then Im not about to start explaining it all.

different people have different views on what makes a seperate installation, but in general most people accept that a shared earthing system, shared supply system, and shared OCPD would amount to the same installation,.

 

[ADS]

:shakehead if you are unable or unwilling to grasp the concept of a seperate installation then Im not about to start explaining it all.

different people have different views on what makes a seperate installation, but in general most people accept that a shared earthing system, shared supply system, and shared OCPD would amount to the same installation,.

But the flats in the Ops scenario don't share an OCPD - they are run from sub-mains each on their own OCPD.

And as for shared supply and shared earthing system - that would mean the block of flats next door is the same installation as well.

'Electrical Installation' - An assembly of associated electrical equipment having co-ordinated characteristics to fulfil specific purposes.

Fits the description of the flat perfectly if you ask me.

 

[Andy™]

But the flats in the Ops scenario don't share an OCPD - they are run from sub-mains each on their own OCPD.

but if the submains are fed from a private DB (and so from 1 OCPD), then it could all be classed as 1 installation

equally, it could also be considered as 1 installation being the submains, and each other DB being its own instllation

 

[steptoe]

But the flats in the Ops scenario don't share an OCPD - they are run from sub-mains each on their own OCPD.And as for shared supply and shared earthing system - that would mean the block of flats next door is the same installation as well.

Fits the description of the flat perfectly if you ask me.

the supply is determined at the DNO's service head,

are you saying next door is off the same service fuse then?

the DNO supply the OCPD for the installation,

now, if you wanted to start TTing some of the flats your stance may have some ground to be classed as a different installation,

until you can fully understand grown up concepts regarding electrical installations to BS7671, I suggest you go back to merchandise replenisher status.

 

[Steve3948]

I would say that anything after the Service Head and Meter is "The Installation", pull the service head fuse and that installation is dead.

That's how I see an Installation.

 

[ADS]

I'll writethe definition out again then, shall I - for those who are having trouble reading English:

'Electrical Installation' - An assembly of associated electrical equipment having co-ordinated characteristics to fulfil specific purposes.

Nothing to do with supply.

Nothing to do with 'shared earthing systems'

Nothing to do with DNOs.

Nothing to do with OCPDs.

If you can't grasp that, I can't help you.