caravan park mains distribution problems

[paul]

A caravan site I have recently worked on had a switch room adjoining a sub station, in the switch room the circuits feeding a large site are terminated in 8 x 200 amp three phase and neutral Glasgow switch fuses, these are marked in pairs as rings going east, west, south and north. My job was to isolate cables from a toilet block and reconnect them in a fibreglass cabinet as the toilet block was to be demolished, further investigation found that this was part of the north ring which fed 3 other buildings and at the switch room was fed by 2 switch fuses one on each outgoing cable (95mm aluminium)however the ring was broken in one of the buildings isolated at 1 of 2 switch fuses which both fed onto a bus bar .In the toilet block was an mccb panel board feeding sub mains to (areas of caravan pitches) which also had to be moved this was fed by 2 cables onto separate mccbs as part of the ring.
After a rush to get the cables moved as demolition had already begun, it came to testing, I asked the maintenance electrician how they had previously been tested and could I see the test sheets as I could see a few issues.
Namely 200amp three phase on 95mm aluminium cable and loop impedance's which would be to high for operation of either the 200 amp cartridge fuses or the mccbs in the panel board as depending on where the ring is separated could give an EFLI reading of between 0.2 and 0.4 ohms which is to high for both.
His answer was that they were tested with the ring made so I explained this meant he was feeding via 2 X 200 amp fuses in 2 separate switch fuses, not good and all other options require rehashing the mains and the introduction of rcd protection which he doesn't want.
Has anyone else come across such a system which was wired by the local electricity authority as was in the day and any ideas what to do about it.

 

[Knobhead]

Pull out of the job now. If you don’t know how a spilt ring works don’t play with it! (You’ll end up in hospital.)

This is why the industry has authorised persons. The DNO are the people to sort it out for you at £1000’s but it will be safe.

Otherwise pay my travel and board + entertainment (bar bill) I’ll come and sort a simple problem out for you. I want a hotel not that death trap caravan park!

I’ve got sequential drawings of how a split ring works. They are perfectly safe IF YOU KNOW WHAT YOU ARE DOING!

Glenn (bless him) phoned me about this. I’ve gone thought the dangers with him for free. I know him, I don’t know you.

 

[Guest55]

this must be some hell of a caravan site for such a heavyweight supply system !!

eek !
lol.

 

[Engineer54]

This system was often used in high rise buildings for enabling isolation of supply to a floor or area, or utilising an alternative supply route. Generally operated in an open ring arrangement, similar to a MV ring.

 

[Knobhead]

The high rise system as is as I recall was two or four risers with cross switched interconnects and the obvious section disconnects at various floors. Bloody great if some numpty forgets where he’s cross connected two risers. This is why for high rise the DNO assumes control, they have the systems in place to deal with it. (So they say!)

I’ve seen this go so horribly wrong on an industrial site, two MV radials paralleled part way along and it wasn’t in the log. It got forgotten in the mists of time until a fault occurred.

 

[ian.settle1]

Pull out of the job now. If you don’t know how a spilt ring works don’t play with it! (You’ll end up in hospital.)

This is why the industry has authorised persons. The DNO are the people to sort it out for you at £1000’s but it will be safe.

Otherwise pay my travel and board + entertainment (bar bill) I’ll come and sort a simple problem out for you. I want a hotel not that death trap caravan park!

I’ve got sequential drawings of how a split ring works. They are perfectly safe IF YOU KNOW WHAT YOU ARE DOING!

Glenn (bless him) phoned me about this. I’ve gone thought the dangers with him for free. I know him, I don’t know you.

Jesus your bar bill will be more than what it will cost for the job to be done LOL!

 

[Specialist]

OP: Obviously I don't know either Tony or Eng54 personnaly, but I do know more than enough about them to be certain that they're both well up on they're game & the advice they've given you is very good advice. For your own safety & that of your customer, please take the advice.

Tony mate: If it was me I'd happily pay your travel & hotel expenses, not sure about the bar bill though lol.

 

[Knobhead]

This is a MV open ring and it shows the operating conditions with the open point at (3). The open point can be moved any where in the system to accommodate faults or maintenance. If a cable needs to be worked on it would be isolated and earthed at each end. This also shows the DNO using the incomer (5) as their open point

LV split rings work the same way without the earthing option. The only time I ever moved the open point was a couple of times to extend the busbars and add a new switch section or to shut a transformer down. Once the work is done the system is then returned to normal operation with the open point at mid way. Basically I’d got two 630A feeds in to the ring from two separate 1000KVA transformers. Each ring fed either three or four sub boards. The danger with our system was the possibility of paralleling two transformers via a 240mm 3½c. Non of the intermediate switches were fused but capable of fault making.

 

[Engineer54]

Yep, that's basically how an MV RMU ring works, and will be protected against paralleling by a Castel Key interlock system, if the two supplies are derived from two different DNO sources. Not so much of a problem as such, if both sides of the ring are supplied from the same source, but still rarely if ever run as a closed ring...

EDIT...
Just had another look at you're drawing Tony, DNO1 supply is providing the source to both sides of the open ring, DNO2 is left doing nothing?? In fact, that source RMU isn't the right configuration for a normal MV open ring system, it's more of an either or set -up.

 

[G1BBO]

Where I work is a similar set up. We have two 66Kv transformer on site which have the option to be in parallel. obviously this link is open because the potential fault current would clearly be tremendous. If I applied my system to Tony's diagram as Engineer54 stated both transformers would be utilized. Think of my system and two transformer supplying one half of each side. You have three series runs which are connected at either end to a transformer making them supplied by both transformers. Somewhere along each series run there is an open breaker ensuring it is only fed from one transformer. So in Tonys diagram 3,4 and 5 would be fed from DNO1 and 1 and 2 from DNO2. The switch on 3 which connects it to 1 would be open giving two series runs. If the cable between 1 and 2 developed a fault and went to earth the breaker on either side would open isolating the supply from either end rendering the faulty cable isolated. To ensure power could still be delivered to 1 the switch on 3 would close so now 1 is fed from DNO1. This is all done automatically. The problem here lies when your HV alarm system doesn't work and you get the board ringing you saying we have detected in excess of 30A in there 66KV neutral. This somewhat comes as a surprise as everything is still on and no phone calls from various department and no alarms. On further investigation looking at the computer you could see the ring had reconfigured itself because of a cable down to earth. Further testing proved one of the cables had gone to earth. Luckily the cable spanned a grand total of 10M across a single road. I don't know if I have been relevant,helpful or just confusing but there you go anyway

 

[paul]

I get the idea of a split ring and that it enables isolation of cables for repair and maintenance my problem is when the ring is split you have 2 radials each 200amp BS88 fused, three phase and neutral on 95mm aluminium cables buried underground and depending on where the break is made an EFLI of at best 0.2ohms and at worst 0.4 ohms this I think is wrong on the rating of the cable and the limiting values of measured EFLI based on the values given in BS7671 also the panel board has mccbs from 63amp to 125 amp which again require lower than the higher measured EFLI

 

[Knobhead]

Yep, that's basically how an MV RMU ring works, and will be protected against paralleling by a Castel Key interlock system, if the two supplies are derived from two different DNO sources. Not so much of a problem as such, if both sides of the ring are supplied from the same source, but still rarely if ever run as a closed ring...

EDIT...
Just had another look at you're drawing Tony, DNO1 supply is providing the source to both sides of the open ring, DNO2 is left doing nothing?? In fact, that source RMU isn't the right configuration for a normal MV open ring system, it's more of an either or set -up.

The intake was on the local DNO MV ring. It's they're choice where the ring is broken, it just happened to be our intake. Fortunately it was an extendable RMU and our 2[SUP]nd[/SUP] feed was tagged on the end so our network could expand.
At another works our intake was again on a ring but normally switched through. Again that’s up to the DNO.

The fun and games start with dual access to switch rooms. The DNO needs access as do we. Normal practice with DNO’s is padlock every operating mechanism, ours weren’t as we may need to carry out emergency switching procedures without looking for keys.

To me a small system is around 3MVA and would be off a local ring. The biggest I’ve worked on was 80MVA with four independent 33KV feeds.

 

[Knobhead]

I get the idea of a split ring and that it enables isolation of cables for repair and maintenance my problem is when the ring is split you have 2 radials each 200amp BS88 fused, three phase and neutral on 95mm aluminium cables buried underground and depending on where the break is made an EFLI of at best 0.2ohms and at worst 0.4 ohms this I think is wrong on the rating of the cable and the limiting values of measured EFLI based on the values given in BS7671 also the panel board has mccbs from 63amp to 125 amp which again require lower than the higher measured EFLI

For gods sake get a trained jointer in. He will know how to bridge the SWA/CPC's without compromising the EFLI

 

[Engineer54]

The intake was on the local DNO MV ring. It's they're choice where the ring is broken, it just happened to be our intake. Fortunately it was an extendable RMU and our 2[SUP]nd[/SUP] feed was tagged on the end so our network could expand.
At another works our intake was again on a ring but normally switched through. Again that’s up to the DNO.

The fun and games start with dual access to switch rooms. The DNO needs access as do we. Normal practice with DNO’s is padlock every operating mechanism, ours weren’t as we may need to carry out emergency switching procedures without looking for keys.

To me a small system is around 3MVA and would be off a local ring. The biggest I’ve worked on was 80MVA with four independent 33KV feeds.

The Dual feed RMU you are showing in your drawing doesn't make the best use of the 2 DNO supplies. Both those DNO supplies should be feeding each side of the open ring. So if you then lose one DNO supply to the ring, the second supply can quickly be switched in to supply power to all of the rings loads. As it stands at the moment, you can only use one DNO supply at a time, in an either or arrangement...

As shown at the moment, the 2 DNO supplies (Position #5) are feeding in to a common bus. Not the best idea, if these two supplies originate from two different distribution networks. The normal supply arrangement for an MV ring main, would be via two separate Switch fuses/ACB'S (with or w/o a bus coupler facility).