Are DNO cables in the UK fused before going into a building? And if so how? What happens if you were to short the cable out before the cutout main fuse?
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Are DNO cables fused?
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It depends on the type and size of service. Normal urban domestic and light commercial supplies are not individually fused outside the building; they are tapped off a distributor cable that might be fused at a pillar or link box. My street, for example, has ~150 houses and is fed in two sections, one each side of a crossroads, each fused at 315A with IIRC BS88 part 5, 80kA J-type fuses. E.g.: Lawson 315A JSU
There is a box in the road at each end of each section, with the fuses present in one but removed from the other, to allow for two alternative sources of supply in case of cable or transformer failure. The main is 3-phase but most services taken from it are single-phase, fused at 60-100A within the premises. There are a few 3-phase service cables feeding new builds that will have a multi-service head, plus a few of the larger houses (mine included) which have two of the three phases using the old 3-wire cables from the days when it was a 240-0-240 DC supply.
I don't know under what specific conditions or current ratings an LV service cable to a single consumer would be individually fused. Perhaps one of our ex-DNO guys will chime in with more info.
There is a box in the road at each end of each section, with the fuses present in one but removed from the other, to allow for two alternative sources of supply in case of cable or transformer failure. The main is 3-phase but most services taken from it are single-phase, fused at 60-100A within the premises. There are a few 3-phase service cables feeding new builds that will have a multi-service head, plus a few of the larger houses (mine included) which have two of the three phases using the old 3-wire cables from the days when it was a 240-0-240 DC supply.
I don't know under what specific conditions or current ratings an LV service cable to a single consumer would be individually fused. Perhaps one of our ex-DNO guys will chime in with more info.
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Normally J type BS88 fuses for normal supplies, bigger supplies may be fed from ACCBs.
I think, from memory, UKPN (try to) standardise at 400A 630A or 800A
Any idea of the distributor cable's size (mm2) and the size of the tap (mm2)?
I am wondering 1) how fast a short circuit below the cutout fuse will take to clear 2) will the BS88 fuse blow before any thermal damage to the tap cable.
I like the idea of normally open tie points. Very elegant. I can see why cable faults do not persist in the UK.
You have what are called Lucy cutouts in the UK correct? And links to a UK pedestal by chance?
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Distributing mains are usually either 185mm or 300mm I think (again based on memory from UKPN specs, other DNO's have their own rules)
Service cables are sized according to the size of supply, a typical 100A SPN supply will be 35mm concentric.
DNO's tend to stick to a few select cable sizes rather than using the full available range of cable sizes.
Service cables are protected from overload at by the cutout fuse.
Lucy is a company which makes cutouts, feeder pillars and other equipment. Other brands of equipment are available.
I believe some DNO's use Merlin Gerin's SAIF type feeder pillars,
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I think they are the same fuses just in a different physical package.
BS88 fuses will generally discriminate as long as their is a gap of 2 standard sizes between them.
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My service (tap) cable is 19/.064 paper-insulated lead-covered which is quite typical of its time. 19/.064 is Olde English for 0.0225 sq. ins, i.e. about 14.5mm². It was probably put in for 60A but my fuses are probably 80A BS1361s, possibly 100A, I can't remember. None of the cutouts on this street are likely to be above 100A so will be fine for selectivity with the main fuses. I don't know the size of our main, 0.3 sq. Ins or 185mm² would be typical.
Lucy Electric are a manufacturer of equipment widely used by DNOs for over a century, other main UK brands are Henley and (historically) Isco and BICC.
Distribution cabinet: Lucy 1600A cabinet
Lucy Electric are a manufacturer of equipment widely used by DNOs for over a century, other main UK brands are Henley and (historically) Isco and BICC.
Distribution cabinet: Lucy 1600A cabinet
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I thought ISCO was a W.T. Henley product line?
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Good point!
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Micktw68
Arms
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London area are fused in the local secondary sub station (11kv-415), occasionally fused in the link box in the street .Not many pillars in London as most of the secondary subs are indoors (small block buildings ) Or package subs - link to SP doc with some photos of current pillars and subs . Interesting photos of open LV boards which are still in use. (skeltag “suicide” board My personal favorite. ) https://www.spenergynetworks.co.uk/userfiles/file/OPSAF-12-007.pdf
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In my post #9, for "19/.064" please read "7/.064". I was writing a report on something wired in the larger cable and have 19/.064 on the brain now. 7/.064 is 0.0225 sq. ins.
Open frame distribution panels like that remain pretty normal at 400v for most dno I think, the protection from contact is keeping the substation door closed.
In circ 1978 I had to carry out my first "real" live switching at 6.6kV - it was an open panel similar to that 400v one, but at 6.6kV - officially one had to stand 2m away and pull each phase separately (just like expulsion fuses on ohl).
However, the substation was under Picadilly train station in Manchester in a room less than 2m wide!
Scariest thing I have ever done in my life!
In fairness every live line switching after that was a piece of cake in comparison.
The engineers were dreadfully disappointed when the station was upgraded and proper switchgear put in place - it had been the test of new engineers for as long as anyone could remember.
The issue really is people breaking into substations, thinking it's just the same as any other place, that's why dno are moving away from substation buildings towards package substations, or fully enclosed panels in bigger substation buildings
Micktw68
Arms
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Generally it was old pennywhistle conduit that would drop off 8the ceiling at one end when a saddle Screw rotted away and swing down and arc out across the LV board.
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Fuses are installed for overcurrent protection, the same as in an installation.
The basic idea is protect against faults causing damage to equipment, starting fires, people getting injured etc.
Why wouldn't you fuse things in the suppliers network?
Others will know better, but I expect it comes down to:
- Fuses are very good at dealing with rare faults (both in term of energy limiting & selectivity with downstream fuses)
- Fuses are cheap
- Fuses are reliable
- A removed fuse is a very good means of isolation (just don't do it on-load, m'kay?)
Don't know, ask Con Edison
In the US DNO fusing is very rare. It just isn't done. This is what happens when our secondaries short circuit:
Only thing stopping the wire from instantly vaporizing is the high loop impedance.
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Don't know, ask Con Edison
In the US DNO fusing is very rare. It just isn't done. This is what happens when our secondaries short circuit:
View: https://www.youtube.com/watch?v=4nLcjDmGr6g
Only thing stopping the wire from instantly vaporizing is the high loop impedance.
Who is Con Edison? Is he a relation of T Edison?
That video sums up pretty well why the DNO install fuses in their network.
I'm a little shocked that the US doesn't have protection installed to prevent things like that happening.
I worked for a DNO for many years, it varies from company to company but our standard distribution for a residential area would be 400A fuse, 185mm cable and a teed off 25mm service ending in a cutout with a 100A fuse.
One thing that is important to understand is the role of the fuses. The 400A fuse at the substation protects the cable from faults phase to phase and phase to earth but the overload protection is provided by the cut out fuse.
So the service cable in your garden has its fault protection in the SS but the overload protection is in the cutout.
The purpose of the cutout is to provide a point of isolation for changing the meter and to stop the premises from drawing too much current. Protection of the customers tails is incidental.
Open LV boards are fine as long as you are careful. I prefer skeltag boards as you bolt the fuse in and then close the link smartly with a pole.
Most boards you have to hold the fuse carrier in your hand, insert the bottom contact and hinge it close. Very interesting when locating faults, if it starts rumbling and the transformer dances about DO NOT remove the fuse, hold it in till the fuse blows or the fault clears.
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One thing that is important to understand is the role of the fuses. The 400A fuse at the substation protects the cable from faults phase to phase and phase to earth but the overload protection is provided by the cut out fuse.
So the service cable in your garden has its fault protection in the SS but the overload protection is in the cutout.
The purpose of the cutout is to provide a point of isolation for changing the meter and to stop the premises from drawing too much current. Protection of the customers tails is incidental.
Open LV boards are fine as long as you are careful. I prefer skeltag boards as you bolt the fuse in and then close the link smartly with a pole.
Most boards you have to hold the fuse carrier in your hand, insert the bottom contact and hinge it close. Very interesting when locating faults, if it starts rumbling and the transformer dances about DO NOT remove the fuse, hold it in till the fuse blows or the fault clears.
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Removing LV fuses on load is the normal method of isolating a LV cable. Try not to do it with fault current flowing though.
Con Edison is the name of utility which serves NYC- the sputtering manhole in the first video took place in Manhattan.
Con Edison - Powering New York City and Westchester
Providing electric, gas, and steam to NYC and Westchester. Pay your bill, manage your account, report an outage, and learn how to save energy.
Though its not fair to single them out because most power companies in the US do not fuse their secondaries.
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