My Interesting problem

[Manator]

I have a very interesting problem, Sidewinder may be able to help knowing that he has worked to HTM 06. I am working in central london (Westminster), on a very famous street actually, the installation is medical, and will be wired to at least 3 regulations although with the new amd, HTM 06 has almost been copied to BS7671.

My problem is that the medical center is on the first floor, and is supplied by 3 consumer units, each supplied via 10mm cable (T&E). Cable runs are approx 7m, 12m and 16m. the building is listed, and that is internal, as well as external. Now I know I can use all 3 consumer units to supply and distribute the power requirements, but my problem will be to keep within the disconection times and touch voltages required. I know what I need to do, but I thought I would throw this problem open to the forum as a kind of open thinking, to see if anyone can come up with anything I have not thought of.

On a side note, whilst the amd does not come into effect until Jan 2012, as this is a medical center and NHS, it will have to comply with medical locations HTM 06, and meigan.

Maximum demand for each consumer unit will not exceed 60amp.

 

[Sidewinder]

So that may be a street with part of the name of a motor cycle in it then!

OK, I'll have a think after my mornings gardening.

Oh then its quali, um later on I promise ]

 

[Manator]

I have only checked 1 consumer units Ze at the moment and had a reading of 0.22, I would have to do a continuity check on the supply cables. The supplies to all floors are via unfused 60amp Glasgows. One consumer unit will do two surgeries and the plant room, another consumer unit will do reception and another surgery, and the last consumer unit will do the OPG and decontamination room, as well as the toilet. I have first fixed one of the surgeries and placed an earth referance bar, all sub bonding to sockets chair and lighting is via 6mm bonding,I intend to use 10mm to the MET, but I am thinking I may have to run 25mm up from the main MET to get the Zs down on all consumer units. I do not think 16mm will cut it.

 

[batty]

Not done any calcs but was thinking of my head 10mm really would not be up to taking 60 amps especially TW/E.

 

[Evans Electric]

So that may be a street with part of the name of a motor cycle in it then!e ]

Ah Royal Enfield Street

---------- Post Auto-Merged at 00:14 ---------- Previous post was made at 00:06 ----------

That was a stupid reply it is of course.....Vellocette Street

 

[Manator]

Ok a little update for you all. It is impossible to do anything to the supply cables, all supply cables are 10mm T&E and are protected at source by BS1361 type II 60 amp fuses.

Because medical locations need to have a disconnection time of under 0.2 seconds the length of cable runs allowed are either shorter, or require a larger cpc than line conductor.

What I did to compensate for this is to use 6mm earth bonding direct to each DB. Each socket ciruit has been limited to 6 outlets, each are earthed via the cpc and an additional 6mm direct earth bond.

When tested I was getting R1+R2 readings of around 0.04 for each circuit (all circuits installed have been 2.5mm T&E radials).

Disconnection times have been verified by testing and calculation.

I am going to love showing this job for an assessment, there is so much wrong with the installation yet so much is right.

 

[steptoe]

gissa job manator?

has to be better than doing this monkey see monkey do rubbish Im stuck with now,

and almost being forced to do the BARE minimum required to comply with the regs :_|

 

[Manator]

To be honest I doubt you would want a job with me, the pressure is so great. This week for instance I ordered all the materials on Monday for delivery Tuesday and Wednesday, they came at 13.10 Thursday, and 14.00 Friday. The job was to be finished for Friday! So I had from 14.00 hrs Friday afternoon to complete the second fix which involved 3 db changes, with additional circuits installed plus all the testing. I did manage 2 db replacements and testing in 4 hours. I had to work Saturday morning to complete the rest of the works, but failed miserably and will have to return.

 

[steptoe]

probably not,

Im getting too old and lazy for the stress,

although on the solar lakes job , which HAD to be complete by the saturday 2pm, I was still waiting for the cable to be delivered at 10am on the Friday,

and when I say cable, I mean almost all of it!!!

gawd I hate it when someone else surveys a job and doesnt order the stuff, and when they do its completely wrong headbang

 

[Robin Spark]

Hi Manator,

I am trying to improve my knowledge on medical locations and have been reading my new OSG, BGB, GN7, breifly looked at Meigan and some other stuff on internet, which is probably why I have now had sore eyes and head for the last few days!

Therefore I hope you dont mind me asking these questions

1. Did you install a propietary made ERB or did you make up something yourself?

2. what OCPD's were in each consumer unit as my understanding if they are 60898's then normal efli values apply down to 0.1seconds disconnection times so you wouldnt need to increase cpc csa or reduce cct length.

3. is it a group1 or 2 medical location?

thanks

 

[Chr!s]

Are you looking for ways to achieve the touch voltage requirements and disconnection times?

Regards Chris

 

[Evans Electric]

Norton Street ?..........BSA Street ?....Triumph Terrace?......Matchless Mews?.......Vincent Parade?....... Aerial Avenue ?

No I give up , what can it be ? :C

 

[Manator]

Hi Manator, I am trying to improve my knowledge on medical locations and have been reading my new OSG, BGB, GN7, breifly looked at Meigan and some other stuff on internet, which is probably why I have now had sore eyes and head for the last few days!

Therefore I hope you dont mind me asking these questions

1. Did you install a propietary made ERB or did you make up something yourself? I make my own, they should be visible, however for most surgeries I do they do not like them to be "on show".

2. what OCPD's were in each consumer unit as my understanding if they are 60898's then normal efli values apply down to 0.1seconds disconnection times so you wouldnt need to increase cpc csa or reduce cct length.There are other matters that need to be adressed, the number of sockets per circuit, and the available circuit length.

3. is it a group1 or 2 medical location?This can vary depending on the type of treatments carried out.

thanks

Good questions my answers in red.

Whilst the update now has the medical locations included, and is almost word for word like the HTM documents, there are still other types of conciderations to be taken into account. I make my own reference bars by using earth blocks inside a single or double gang back box with a blank plate to cover them. each bond should be labelled and the cover has a label to indicate its purpose.

@ Plumber

I have never had a problem with disconnection times, touch voltages can sometimes be a problem, in larger installs but seldom. The main problem is that most installs will require a maximum Zs value for a circuit, because the cpc is often smaller than the Line conductor additional bonding is required to extend a circuit.

Like I have said I seldom have a problem, the main problem with the London job was the type of distribution, and the earthing arrangements, this posed a problem not only to get the required Zs measurements, but the conformity to regulations, the EIC for this job had 14 additional pages for remarks and deviations.