500kva supply and distribution ct extension

[mike the sparky]

Hi guys
Its been a while since i have been here and i have a new problem.
I am to supply and install a 500kva distribution over 2 stages, 250kva now and the rest in the next 5 years. I am going to use Eaton mem switchgear, a 800a buzbar with 630mm2 tails and a 400a mccb within a 12 way distribution unit. The incomer is a 555mm2 bunched wavecon cable fed directly from the substation 3 metres away. The only fuses are at the sub and my 400a mccb
I have been asked to supply a ct extension chamber by my local electricty board which i thought was their equipment. This is proving to be difficult and time consuming.
I feel that it is being made difficult by a third party not releasing enough information.
Any information would be appricated.

 

[netblindpaul]

mike,
Don't do anything unless you have enough info to do everything as you are the designer under CDM and this will come back and bite.
Oh and by the way, it is actually kVA!

 

[Knobhead]

It’s quite normal for the DNO to ask for a metering chamber. Don’t ask me why you should pay for it but….. I’ve installed many boards with metering chambers but they were supplied from our own MV system so no arguments about metering security. Only once did I get mixed up with a DNO direct LV feed, what a pain! As Paul said get the ground rules drawn out first, otherwise….. Re the incoming MCCB, I hope you’ve got one that can be easily upgraded to 800A, if not your going to have a lot of work at some time in the future.

 

[Engineer54]

Use an 800A frame with a 600A adjustable MCCB to give you a 400A setting, or an electronic adjustable 800A MCCB that can be adjusted down to give you a 400A setting. As Tony stated fail to plan for the future, with ready made panels at your own peril, ....Breaker Frame sizes, are generally not interchangeable. it'll be like knocking a square peg into a round hole...

 

[mike the sparky]

Thanks for your help. This is the first board, a second will be installed at a later date 2 - 3 years away, the same spec as the first. This does bring up another question:
If the main fusing is at the sub station, i will need to isolate the buzbar to install the second board. Can this supply be switched off at the sub or will i need to install an 800a isolater after the ct chamber.
I have installed a few 250KVA installations but not so with the 500KVA.
Great site, cheers for all your help

 

[Knobhead]

Have a look at http://www.eon-uk.com/downloads/Notes_of_guidance_positioning_of_commercial_connections_v1.pdf

 

[Engineer54]

Thanks for your help. This is the first board, a second will be installed at a later date 2 - 3 years away, the same spec as the first. This does bring up another question:
If the main fusing is at the sub station, i will need to isolate the buzbar to install the second board. Can this supply be switched off at the sub or will i need to install an 800a isolater after the ct chamber.
I have installed a few 250KVA installations but not so with the 500KVA.
Great site, cheers for all your help

You should have a means of protecting and isolating the LV side of the transformer. If this transformer is feeding 2 switchboards there must be a front end isolator/switch fuse/ACB. Normally this is incorporated in a main switchboard, that will supply other switchboards. ie .... In your instance a 800A Main breaker, 2 X 400A sub-main breakers. There are many variations in the configuration of said switchboards, but all will incorporate those 3 main breakers...

What your describing is 2 switchboards without any means of a central or main isolation point, which isn't very good practice, if not illegal.... These is little or no difference in installing a 500KVA installation to a 250KVA, or come to that a 1000KVA system, only the protection levels change and maybe the size of the switchboards/panels...

 

[Knobhead]

As far I know a central isolation point is a recommendation but not a requirement. I hope so otherwise I’ve cocked up several times. You can have two independent supplies but they must be interlocked if you’re going to incorporate a bus coupler. This has to be with the agreement of the DNO. You have no option but to speak to the DNO, they will set out the requirements for metering and protection. Don’t try to second-guess them because they will bowl a googly at you. Have you had a look at the link I gave?

 

[Engineer54]

As far I know a central isolation point is a recommendation but not a requirement. I hope so otherwise I’ve cocked up several times. You can have two independent supplies but they must be interlocked if you’re going to incorporate a bus coupler. This has to be with the agreement of the DNO. You have no option but to speak to the DNO, they will set out the requirements for metering and protection. Don’t try to second-guess them because they will bowl a googly at you. Have you had a look at the link I gave?

That is only required when you have 2 supplying transformers to a single switchboard.

Were talking about a single transformer supplying 2 separate switchboards, it needs a form of front end protection for the supplying transformer. It's not on, having 2 X protective devices, or having to switch 2 devices in order to fully isolate the supply transformer. I never seen such a set up on any installation i've seen or worked on.

I hope Mike has taken account and provided a switchboard with the appropriate fault bracing/rating that the 500KVA can deliver under fault conditions. A fairly common failing of occasional designers of transformer main panels/switchboards... Especially when having 2 supplying transformers feeding a common switchboard, even with a bus coupler...

 

[Knobhead]

E54 I meant by interlocking was that if you need to close the bus section only ONE feeder can be used. Unless you’ve got my Castell override keys!

 

[Engineer54]

E54 I meant by interlocking was that if you need to close the bus section only ONE feeder can be used. Unless you've got my Castell override keys!

You know, i haven't used a castell key system on panels for years now. All the switchboards i've worked on for a good number of years now (MV/LV) have all been fully automatic. About the only castell interlocking i have been using, is on RMUs and transformer security access gates.

 

[Knobhead]

Still in common use. One reason I like them is it can be seen that a key is exchanged. Transformer access we’ve all ways used a plain padlock, but we’ve never needed access to transformers with exposed HV bushings.

 

[Engineer54]

Oh they are the best thing ever invented to control manual operated kit, saved more than just a few lives over the years. But totally useless on a fully automatic system.

None of those transformers in the cages had exposed bushings, they just didn't want unauthorised persons anywhere near energised transformers, doing something silly!! lol!!

 

[Knobhead]

Just thought about my statement about transformer pens. Yes we did use Castell keys for them but they were a bit specialised, between 60 and 120KV transformer rectifier sets. If anyone tried to defeat the key sequence for then they deserved to die!

 

[mike the sparky]

I think i'm there now. Ryefield will supply a ct chamber with an 800a MEM cb 50 kA, bolted to an 800a busbar. 300mm tails to one 400a mccb on a 12 way distribution board. A second 12 way 400a board can be fitted in a few years. Again, thanks for all your help.

 

[Knobhead]

Thought the board would provide the LV protection. It is in that link I gave.

Let us know how it progresses, I for one am interested.

 

[mike the sparky]

Cheers Tony. That link didn't connect, tried it a couple of times. Would still like to check it out at some time.

 

[Knobhead]

Sorry, try
http://www.eon-uk.com/downloads/Notes_of_guidance_positioning_of_commercial_connections_v1.pdf