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?