big headache

[jim23]

hi

discovered a problem at the company where i work and would like some advice on possible solutions please so i know what im looking at before i go and tell the boss!

what we have is a 3 phase supply that goes through a fused isolator (3x800amp bs88 fuses). after this it splits via a bb chamber and feeds 3x200amp db's, 2 office supplys(45amp each), a 30amp sub main and another db at 300 amp.

there is also a bussbar fed directly from the main fused isolator which is wired in swa 4 core 120mm. the bussbar i believe is rated at 300 amp and has 10x30 amp isolators and socket outlets on it.

i think your getting the theme here that im a bit concerned the whole installation is overloaded! ( this came about as its been suggested that some more points be added from the bussbar to power a 100 amp machine!)

thing is what is the 120mm cable (for the bussbar) rated at? sureely its not sufficient to be protected by 800amp fuses?

is it time to take a trip to the boss's office to tell her she should be concerned?

 

[RF Lighting]

The 120mm² 4 core SWA feeding the second busbar chamber is rated at 290A. This is assuming that it is installed to reference method 11, which by the sounds of the installation, it probably is.
So this cable is running pretty much at its maximum capacity.
Also this is assuming that the cable is sized for current carrying capacity, and not to overcome voltsdrop etc.
However this cable should be fed from a switch fuse off the first busbar, and not fed straight off the busbar.


What is the rating of the first busbar chamber? (the actual chamber, rather than just the isolator fuses)
What size is the incommer?
How well loaded up are the TP DBs?

You may have capacity for the new machine, dependant on diversity but it does sound like it is running pretty close to the wire.

Have you tried putting a clamp meter on the supply to the first busbar, when the (presume) factory is fully running, remembering there may be more load in winter with heating.

Hope this answers some questions but feel free to ask more.

 

[Spark123]

The rating of the 120mm² cable depends on the type of cable i.e. 70deg C or 90deg C and how it is installed, i.e. if it runs through thermal insulation, ambient temp, grouped with other cables, run on tray plate or clipped direct to a non-metalic surface. 120mm² cable not rated anywhere near the full 800A may still be installed if overload protection is installed and the cable passes the adiabatic equation, although overload protection is doubtful as the busbars can be quite easily added to. I would therefore argue the point that the cable should have suitable fuses fitted at source to provide protection.

 

[jim23]

thanks for the input guys, much appreciated.

ill try and make the picture clearer.

the mains tails are i believe 185mm. these go to a 800A main isolator that has 3x800A fuses in for each phase. from there it splits off to power the high level bussbar wired in the 120mm 4 core swa( this has the previously mentioned equipment fed from it). also fed from the main isolator is the bussbar chamber (rated at 800 amp i believe) which then feeds all the other mentoined stuff.

all the db's are pretty full up. some have 400 amps worth of equipment running from them( and they are only rated at 200-300A!).

not sure how to do the diversity for three phase but common sense is telling me that the supply is already running pretty much to capacity hence the concern.

 

[Steve]

are the cables warm? in fact, stand by the door, can you feel the heat? Has the insulation melted off the cables yet???

 

[Spark123]

A starter would be to get an electrician to do a PIR (periodic inspection report) on the premises.

 

[jim23]

ha ha crafty! think if they put the new machine in ill book the day off when there givin it the trial run.

believe it or not they had a pir done last year but he must have been a bit of a clown because none of this was mentioned in the report!

he has the main fuses down as 400A but they are definetly 800A because i checked it out.

 

[Spark123]

Did you read 800A off the endcap/the body of the fuse? 400A is still a bit too big to protect a 290A cable.

 

[jim23]

off the body of the fuse yeah, 800A bs88's!

 

[fattony]

This is kind of related but does not answer the origional post, sorry.

If the suppliers come from the transformer and wire down the road red, yellow, blue all the way down the street, (say the street has 300 houses on it, 100 houses on red, 100 houses on blue and 100 houses on yellow) and each has a 100A supply.

would they use a cable that could carry 1000A or do they use diversty what size cable could carry that, or is it a smaller cable not copper that has a higher current carrying capacity?

Moving on to a 3 phase supply, there are sometimes led's so you can see the phase is live - these are wired from the load side of the main switch but with 1mm or smaller. If the isolater is 250A how is this ok - is it that the LED's wont draw much current so the cable size is relevent?

Just been thinking, maybe I should stop.

 

[breezer]

Moving on to a 3 phase supply, there are sometimes led's so you can see the phase is live - these are wired from the load side of the main switch but with 1mm or smaller. If the isolater is 250A how is this ok - is it that the LED's wont draw much current so the cable size is relevent?

Just been thinking, maybe I should stop.

i bet they are not leds, since leds dont like ac, more likely they are neons with current limiting resistors, and it wouldn't surprise me if they were actualy connected to torodial coils that each phase cable actualy runs through

 

[RF Lighting]

This is kind of related but does not answer the origional post, sorry.

If the suppliers come from the transformer and wire down the road red, yellow, blue all the way down the street, (say the street has 300 houses on it, 100 houses on red, 100 houses on blue and 100 houses on yellow) and each has a 100A supply.

would they use a cable that could carry 1000A or do they use diversty what size cable could carry that, or is it a smaller cable not copper that has a higher current carrying capacity?

They would apply diversity to allow them to use a cable rated at say 5000A (figure plucked from thin air) (Oh and BTW 100 houses @ 100A is 10,000A) but the fuses would also reflect this so that if all the houses drew their 100A the 5000A fuses would blow, protecting the cable.

Moving on to a 3 phase supply, there are sometimes led's so you can see the phase is live - these are wired from the load side of the main switch but with 1mm or smaller. If the isolater is 250A how is this ok - is it that the LED's wont draw much current so the cable size is relevent?

Just been thinking, maybe I should stop.

They should really have a small fuse in-line, incase they went short circuit, but they will not overload the 1mm cable as they draw a small amount of current, and are the only thing connected to the cable.

 

[Pensdown]

Moving on to a 3 phase supply, there are sometimes led's so you can see the phase is live - these are wired from the load side of the main switch but with 1mm or smaller. If the isolater is 250A how is this ok - is it that the LED's wont draw much current so the cable size is relevent?

Just been thinking, maybe I should stop.

i bet they are not leds, since leds dont like ac, more likely they are neons with current limiting resistors, and it wouldn't surprise me if they were actualy connected to torodial coils that each phase cable actualy runs through

They are LED's. If you have browse around RS they come in all AC voltages from 24 -240. I assume they have a built in rectifier.

Some people use in-line fuses but often the cable is the fuse as it's so short.

 

[jim23]

cheers for the input guys.

can i have some opinions of what people would do in this situation? i know the circuits already working to its max and theres more than enough equipment on there alraedy, it seems a no brainer to me but the boss is a '' it'l bi reyt merchant '' n she's putting the pressure on.

is it a case of stand your ground and use the excuse that the circuits not protected by a high enough rated fuse?

 

[Adam_151]

If the suppliers come from the transformer and wire down the road red, yellow, blue all the way down the street, (say the street has 300 houses on it, 100 houses on red, 100 houses on blue and 100 houses on yellow) and each has a 100A supply.

Believe it or not, they assume about 2kw per house, because thats what it averages out to for a large family house, and it works* (work out the average from your leccy bill**) gross overloads of many times the rateing for about half an hour are tolerated because of the high thermal capacity of all that oil and metal in the transformer, and the supply cables are burried in the ground (and heavy demands most often co-incide with cold weather...) The average street main is 95mm² I believe, I don't know what they rate it at (somewhat higher than BS7671 I guess) or how many houses are hung off it

*Of course it doesn't quite work out if you have large lamps in your attic...