BEMTs

[johnboy6083]

hi lads/lasses.

tommorrow im going to finish a job in a workshop, that the company i work for have recently acquired. Our install was only data/telephone and power in dado trunking. Im expecting that im goning to be asked to test it, which isnt a problem.

The problem is with the bonding. the whole building is fed by a 100A 3ph PME supply. supply goes into a 3ph DB, which feeds the workshop/warehouse area, and the submain to DB2, which is a 1ph Cu for the front of house/offices.

The water enters the building directly behind the CU, and the gas is 3m away also.
The bonding size is currently 6mm, and needs updating to 10mm. Im not sure where the bonding goes to, ie DB1 or DB2, but how do you go aobout creating a BEMT?

What are the limitations, ie does the cable size have to be a certain size, or does it go on the max size of bonding tobe connected to that BEMT? could i connect both bonds to DB2 (fed by 16mm2 SWA)?

Thanks in advance for all replies


John

 

[johnboy6083]

bumping the post!

 

[Adam W]

Sorry what's a BEMT?

 

[Chr!s]

The problem is with the bonding. the whole building is fed by a 100A 3ph PME supply. supply goes into a 3ph DB, which feeds the workshop/warehouse area, and the submain to DB2, which is a 1ph Cu for the front of house/offices.

If the bonding is connected to the bemt in the cu, then from origin to cu would require a bonding conductor sized based on the neutral csa, which may be 10mm as you've said.

The cpc may also act as a combined cpc/bonding and needs to be sized to the greater of the two requirements.


Chris

 

[johnboy6083]

Sorry what's a BEMT?

building earth marshalling terminal

 

[johnboy6083]

could i use the structural steel for the same purpose? (hypothetical, as id rather not)

There are purlins running the full perimeter of the building, and they are all bolted together. Could i bond all services to that, and then bond that to MET with largest of the bonding size cable?

 

[Chr!s]

could i use the structural steel for the same purpose? (hypothetical, as id rather not)

If the steel work is extraneous then it will require bonding, providing the steel work is of a suitable size etc, i see no reason why not.

 

[johnboy6083]

can you point me in the direction of any books, websites ect, so i can brush up on this? i dont like not knowing anything.

Cheers

John

 

[Engineer54]

''could i use the structural steel for the same purpose? (hypothetical, as id rather not)

There are purlins running the full perimeter of the building, and they are all bolted together. Could i bond all services to that, and then bond that to MET with largest of the bonding size cable? ''​

NO, ...but if the building is a steel framed building, then the frame needs bonding to the MET. The idea behind METs and EMTs is that each of the bonding cables are disconnectable for testing purposes (among other things). If you connect all the other service bonds to one of the bonded items, then ALL must be disconnected in order to test that item. Which is why it is always better to have a separate MET and EMTs. Although it is still permissible to use the DB or CU earthing bars as a EMT and/or MET....

 

[Chr!s]

NO, ...

No? Why not, providing the steel work meets the requirements of BS 7671?

but if the building is a steel framed building, then the frame needs bonding to the MET. The idea behind METs and EMTs is that each of the bonding cables are disconnectable for testing purposes (among other things). If you connect all the other service bonds to one of the bonded items, then ALL must be disconnected in order to test that item. Which is why it is always better to have a separate MET and EMTs. Although it is still permissible to use the DB or CU earthing bars as a EMT and/or MET

Bs 7671 allow the use of extraneous conductive parts as a protective conductor 543.2.6

As for testing GN3 p 35 paragraph 4 testing between extraneous conductive parts.

 

[Chr!s]

The idea behind METs and EMTs is that each of the bonding cables are disconnectable for testing purposes (among other things)

Well a electrical installation has only one met, the emt is a means of connecting extraneous conductive parts to the met, this allows us to bond multiple items in a location, and effectively bond them back to the met via a single conductor.

 

[Engineer54]

Correct, but you need to be able to disconnect individual bonding cables whether at an EMT or the MET. How do you disconnect the steel frame of the building, with all the other bonding cables connected to it?? You can't, which is why a bonding cable from the steel frame to the EMT or the MET along with all the other bonding cables from the other services, is the way to go.... Then you are in a position to be able to disconnect individual cables for testing etc....

 

[olly w]

Correct, but you need to be able to disconnect individual bonding cables whether at an EMT or the MET. How do you disconnect the steel frame of the building, with all the other bonding cables connected to it?? You can't, which is why a bonding cable from the steel frame to the EMT or the MET along with all the other bonding cables from the other services, is the way to go.... Then you are in a position to be able to disconnect individual cables for testing etc....

I don't mean to be picky but how would you effectively expect to remove individual bonding cables from a steel structured building?

i.e. The incoming SWA gets terminated into a steel panel board that is bolted directly to the steel structure of the building.

The water and gas pipes are all bolted to the same steelwork.

The structural steelwork itself is an earth mat.

Mezzanine flooring with all types of equipment carry parallel paths across all services.

So will removing a main bond really have the desired effect?

 

[Engineer54]

Steel framed doesn't necessarily mean the building is All steelwork, ...normally only the columns and the roof structure. They can be block or brick or any other infill between the columns.

Doesn't matter what the structure is made of, you will still be required to individually bond the various services etc to EMT/METs and still be able to disconnect individually. But as you imply, some steel framed buildings can at times play havoc for testing purposes when things are getting parallel paths all over the place... But as for the installation of bonding cables, ...thems the rules!!!

 

[Deleted member 26818]

Why would you require to disconnect bonds individually?

 

[Chr!s]

Why would you require to disconnect bonds individually?

as far as i am aware theres no requirement to disconnect structural bonding

 

[Engineer54]

Hmmmmm.....!! Perhaps we should weld all these bonds together and have done with it then!! Makes you wonder why we have connectible earth bars, and why some of the bigger one's have multiple removable isolation links, perhaps we ought to weld them up too!!!!

 

[Deleted member 26818]

You could weld the bonds together, I believe that is an acceptable methode as per BS7430.
I thought the purpose of those removable links was to measure Ze without having to remove the bonds individually.

 

[Engineer54]

Not as far as i'm aware , when the bonds are coming from different building services. Multiple removable link sections on earthing bars are not there ''Just'' for measuring Ze!!! ...You would only require one removable link if that was the sole purpose..

What are you saying here in answer to a question on another thread on bonding conductors? Am i missing something along the line? ...It is a bit late over here...lol!!!

b) The purpose of this test, is to ensure that the conductors are continuous, to that end they will have to be disconnected at each end so as there are no parallel paths.

 

[Chr!s]

Hmmmmm.....!! Perhaps we should weld all these bonds together and have done with it then!! Makes you wonder why we have connectible earth bars, and why some of the bigger one's have multiple removable isolation links, perhaps we ought to weld them up too!!!!

No need, you just carry out tests to ensure continuity.

A distribution board mounted next to a RSJ, it is acceptable to connect the RSJ to the met and use the RSJ as a bonding conductor, you may connect extraneous conductive parts to that RSJ further down in the location, providing the RSJ meets the requirements of bs7671.

Not sure where your issue lies.

 

[ian.settle1]

building earth marshalling terminal

WTF that? is it a new term that is coming out in the 1st amendment

 

[Deleted member 26818]

No it's quite an old term.

 

[Engineer54]

]No need, you just carry out tests to ensure continuity.
[/B]
A distribution board mounted next to a RSJ, it is acceptable to connect the RSJ to the met and use the RSJ as a bonding conductor, you may connect extraneous conductive parts to that RSJ further down in the location, providing the RSJ meets the requirements of bs7671.

Not sure where your issue lies.

So you would have maybe main water and gas main supply, along with other bondable building services connected to an RSJ scattered along it's length?? Not on any project i was involved with you wouldn't!!! What's that all about, ....hunt for/find the the bond connection, ...oh it's somewhere up there, along that RSJ mate, .....Bloody Ridiculous!!!

Oh and how are you going to know those test are confirming continuity?? Very difficult at the best of times, in steel frame buildings that are made up almost exclusively from steel/metalwork!!

 

[Chr!s]

So you would have maybe main water and gas main supply, along with other bondable building services connected to an RSJ scattered along it's length?? Not on any project i was involved with you wouldn't!!! What's that all about, ....hunt for/find the the bond connection, ...oh it's somewhere up there, along that RSJ mate, .....Bloody Ridiculous!!!

Each to their own, im not saying it would be the method i would employ,in some scenarios it may be the most appropriate way, but to say NO as you did is incorrect, extraneous conductive parts may be used as a bonding conductor, fact.

Oh and how are you going to know those test are confirming continuity?? Very difficult at the best of times, in steel frame buildings that are made up almost exclusively from steel/metalwork!!

How do you think?

 

[Engineer54]

As i stated, .....NOT on any project i was involved with you wouldn't!!! and you can take that to the bank!! lol!!

Not without disconnecting them first, Which was my point from the very start!!!

 

[Chr!s]

As i stated, .....NOT on any project i was involved with you wouldn't!!! and you can take that to the bank!! lol!!

That opinion is a little emphatic., As i stated, it comply s with BS 7671, theres more than one way to skin a cat.

Not without disconnecting them first, Which was my point from the very start!!!

You need to prove continuity, providing the continuity is ok wheres the problem?

 

[Engineer54]

Sorry Chris, ...not going any further with this thread, it's just going round in circles now. Too many what Ifs and what abouts!! lol!!
There are quite a few anomalies and contradictions in BS 7671 past and present, that many pounce on to make a point.

The real point on this particular matter is that any bonding cable at either EMTs or METs should be disconnectable for testing purposes. Whether they will ever be tested in the future, is another argument....

 

[Chr!s]

The real point is on this particular matter is that any bonding cable at either EMTs or METs should be disconnectable for testing purposes. Whether they will ever be tested in the future, is another argument.

There would be a cable at both the met and emt for disconnection, these would be connected to the structural steelwork which is being used as a bonding conductor.