Bonding Required?

[kme]

Okay. Commercial premises being rewired from scratch.

A series of 5 interconnected sheds. Existing was one DNO supply point, subbed to all 5 sheds - but we`re having our own TN-C-S installed.

All OUR water is alcathene; however; the water service enters the first shed in copper, and runs all the way to the end shed - feeding everyone along the way.

The pipe is installed at roof height - 6.5 metres - along the steel girder forming the front of the building.

Our plumber has teed into the pipe, and brought out alcathene to all our services.

We`re bonding the structural steel anyway - but I need to justify installing a 50mm ( or maybe 75mm) main bonding conductor across the whole length of the building - 50 metres.

On the one hand - the water service is IN our building - but inaccessible ( effectively) ; but on the other, the client sees our water service as plastic.

OR - would you deviate, and bond the copperwork to the structural steel adjacent to it? 

Discuss???

 

[steptoe]

I'd bond to the steelwork,

the steelwork will be bonded anyway,

its all extraneous, imho

btw, I dont think its a deviation,

merely bonding extraneous conductive parts.

 

[kme]

Mmmmm

I`ll refrain from commenting yet - until other views have surfaced................

 

[Sidewinder]

Bond to the steel, no issue, no deviation, structural steel is a valid bonding conductor.

 

[Bruspark]

Replace the supply pipe with plastic

 

[apprentice87]

Hi KME

What sort of supply is the original one?? Reason for asking, is that if you have multiple supplies to a steel framed building, only the one can be TNCS, all the others have to be TT'ed..

Reason being; You have ten supplies yes?? all TNCS. Incoming neutrals form the [each] earthing conductor, which is in turn bonded to your shed frame. This means that all the neutral currents for ALL the installations will try to disappear back down which ever particular neutral conductor that happens to have the lowest impedance. The DNO will not allow this sort of thing....

The have a list of other rules that are well beyond my capabilities to understand, but here is a link to them!! http://www.westernpower.co.uk/getattachment/Connections/New-Connections/Supplies-to-Multi-Occupancy-Buildings/SD5C.pdf.aspx

john...

 

[revjames]

just be careful with the rusty water...... :innocent

 

[binky]

:slap :slap :slap

 

[Andy™]

Hi KME

What sort of supply is the original one?? Reason for asking, is that if you have multiple supplies to a steel framed building, only the one can be TNCS, all the others have to be TT'ed..

Reason being; You have ten supplies yes?? all TNCS. Incoming neutrals form the [each] earthing conductor, which is in turn bonded to your shed frame. This means that all the neutral currents for ALL the installations will try to disappear back down which ever particular neutral conductor that happens to have the lowest impedance. The DNO will not allow this sort of thing....

The have a list of other rules that are well beyond my capabilities to understand, but here is a link to them!! http://www.westernpower.co.uk/getattachment/Connections/New-Connections/Supplies-to-Multi-Occupancy-Buildings/SD5C.pdf.aspx

john...

its no different from a row of houses all having TNCS and having a common steel gas pipe

 

[apprentice87]

Hi Andy,

Sounds logical, but that is the DNO for you!!!!

john..

 

[steptoe]

Is this not the reason for MEBs being oversized on TNCS,  so they can carry a fault in your neighbours install that you may import via the extraneous metalwork. 

 

[Sidewinder]

Jon,

I already had a copy of that document as it happens.

I skimmed it again, and I could not see where they specify that the PME earth can only be used for one part of the communal structure, there is, a clause that says all sectors of the communal structure must use the same earthing system though!

I'm not saying that you are wrong, I did not spend the time perhaps needed to study it correctly.

 

[apprentice87]

Hi Steps, Yes, that is right, but after your MEB's the current disappears down the DNO's neutral in the service cable. This is what worries them!!

john..

Hi Paul, Yes you are correct, the rule i am on about is in a different document. I am trying to find it now!!

john..

 

[apprentice87]

Hi Paul,

Cannot find the document i am on about, but see page 11 of this one from UK Power networks. It is close enough!! https://library.ukpowernetworks.co.uk/library/en/g81/Design_and_Planning/Planning_and_Design/Documents/EDS+08+-+0118+Supplies+to+Multi-occupied+buildings.pdf

Here is an extract from what Eon Have to say, [see page 54 and 55 here; http://www.eon-uk.com/downloads/network_design_manual.pdf

[SIZE=11pt]1.3.1.4 Multi occupancy buildings[/SIZE]

[SIZE=11pt]Where two or more customers occupy separate units within the same building there are two options[/SIZE]

[SIZE=11pt]available:[/SIZE]

[SIZE=11pt]1. Group metering position. [[/SIZE][SIZE=11pt]Preferred option][/SIZE][SIZE=11pt]. A single metering position is installed using a[/SIZE]

[SIZE=11pt]single or duplicate service cable (see arrangements A to D below). The developer should[/SIZE]

[SIZE=11pt]provide customer owned cabling from the group metering position to each to each unit. This[/SIZE]

[SIZE=11pt]option has two advantages:[/SIZE]

[SIZE=11pt]a. Lower cost of mains and services[/SIZE]

[SIZE=11pt]b. All earthing and bonding is made at a common point[/SIZE]

[SIZE=11pt]c. [/SIZE][SIZE=11pt]A PME terminal is available for each unit [/SIZE][SIZE=11pt]for TNCS earthing.[/SIZE]

[SIZE=11pt]If two or more service cables are required they must be positioned in the same room and have[/SIZE]

[SIZE=11pt]the same sized neutral conductors. The neutrals of the cut-outs shall be bonded together with[/SIZE]

[SIZE=11pt]120mm[/SIZE][SIZE=7pt]2 [/SIZE][SIZE=11pt]copper conductor coloured green/yellow with blue tape marker at each end to denote that it is a current carrying neutral/earth bond.[/SIZE]

[SIZE=11pt]2. Individual services to each unit. [/SIZE]

[SIZE=11pt]A distribution main is provided outside the building and[/SIZE]

[SIZE=11pt]individual services are connected to each unit. This option may be considered in large[/SIZE]

[SIZE=11pt]buildings where it may be impractical to provide long customer owned cabling from a central[/SIZE]

[SIZE=11pt]metering point.This option has the following disadvantages:[/SIZE]

[SIZE=11pt]a. Higher cost of mains and services[/SIZE]

[SIZE=11pt]b. Earthing and bonding is made at numerous points. Where TNCS earthing (PME) is[/SIZE]

[SIZE=11pt]used much of the neutral current will return via the building’s metalwork and the[/SIZE]

[SIZE=11pt]service cable electrically closest to the substation. This will result in high electrical[/SIZE]

[SIZE=11pt]fields which may affect electrical equipment. More importantly, it may also overheat[/SIZE]

[SIZE=11pt]the PME bonding conductors and the neutral conductors of the service cable[/SIZE]

[SIZE=11pt]electrically closed to the substation. This effect is especially pronounced in metal[/SIZE]

[SIZE=11pt]framed buildings.[/SIZE]

[SIZE=11pt]c. To avoid this phenomenon [/SIZE][SIZE=11pt]PME shall not [/SIZE][SIZE=11pt]be provided to any unit. All units should[/SIZE]

[SIZE=11pt]employ TT earthing. The current edition of BS7671 permits a building’s metal frame to[/SIZE]

[SIZE=11pt]be used as the main earth electrode. [/SIZE][SIZE=11pt]See Central Networks’ Earthing Manual Section[/SIZE]

[SIZE=11pt]E6 for more information.[/SIZE]

[SIZE=11pt]d. Where current transformer (CT) metering is used the metal meter cabinet shall be[/SIZE]

[SIZE=11pt]bonded to the neutral of the cut-out. Any trunking between the metering cabinet and[/SIZE]

[SIZE=11pt]the customer’s equipment shall be plastic or other insulating material.[/SIZE]

[SIZE=11pt]Where multiple LV cables are used they shall originate from the same substation. Providing LV[/SIZE]

[SIZE=11pt]supplies from two or more substations to the same building is not permitted on safety[/SIZE]

[SIZE=11pt]grounds.[/SIZE]

john...

 

[Sidewinder]

KME is an the WPD area though, and as we know DNO's have their own rules!

Not saying your wrong.

WPD are in the process of installing a new supply for the job KME is on, so they should be giving the requirements.

 

[kme]

And they have.....

Their existing ( into a 5 bay commercial/industrial) is a metallic henley box; fed from an underground wavecon 185mm ; which is (below ground) potted onto a 35mm aluminium concentric, which rises to the henley. From there, three metering positions are provided ( tails exit the henley through separate holes, not slit!!).

We want at least 160A/phase; possibly later upped to 315A - so they`re connecting to a "spare" pot end on the local sub ( which feeds the current supply), with "our own" 185mm wavecon. The 60A submain currently taken to the unit will be isolated at source ( although there IS a "communal" lighting cable, in pvc cond., running along the front of the building, adjacent to the water pipe.)

WPD have specified they will provide PME.

The existing install & distro is shambolic mess. Apparently, half a dozen sparks have been there in the last 5 years, looked at it, and left - never to return or answer the `phone again. I don`t intend to have anything to do with the existing supply - we`d destroy the aluminium WPD cable in about 30 mins of running ( the hot water tank in the brewery is fitted with 3 x 12KW TP immersions.........without considering any other stuff!!!!!).

I`m not convinced there are any decent bonding arrangements in place at the origin - and it isn`t an easy spot to access - so I`m not going near that - cos nobody will want to pay for any time I spend on that. As long as OUR unit is compliant - the other units; their faults, and any DNO issues are beyond my control or remit.

As far as I was concerned, I can hit the steelwork with a 50mm MBC, and then strap the copperwork at entry to the building steelwork too......... It appears that the general concensus is that is acceptable?

 

[Sidewinder]

Fine bud. Crack on.

 

[steptoe]

is your 50mm bond large enough?

other than that I see no issue,

but, I stand to be corrected, or I could bend over if SSS is doing the correcting,  ray

Im thinking if you later upgrade to 315A

 

[Sidewinder]

It is for WPD PME requirements Steps, plus I think BS 7671 also!

 

[kme]

If the tails can be 125mm; then yes bud; and I think they can (just)