I don't work on 3-phase very often, so I thought I'd take a chance yesterday to get some photos of a typical British 3-phase service entrance for small commercial premises.



The feeder is underground, tapped from the mains under the street, and enters the bottom of the light gray panel. The pull-outs at the top of that block are cartridge fuses to BS1361, in this case rated at 60A each. Notice that the fuses and cables entering the meter have been tagged for phase identification (red, yellow, blue).



The right hand side of the entrance unit contains the neutral terminal block. You should also be able to see a green/yellow cable entering the block top right. This is a PME (TN-C-S) installation, and it is at this block at which the building earth is bonded to the neutral (unlike in the U.S., ground and neutral are always kept separate in panels).

The label on the service block says:

Here is a typical older style 3-phase switchfuse, this being of MEM make (Midland Electrical Manufacturing):



The handle must be in the off position (as shown) before the cover can be opened. As you can see, this particular unit did not have a neutral wired to it:


The fuses are pull-out porcelain carriers which take old-fashioned fuse-wire. In this unit, they are rated for 20A maximum:



And finally:


As you can see, there is no load on that 3-phase panel now. This shop (very small, about 12' wide, 30' deep) has just been turned into a taxi office, so it now has only a few lights, radio, computer, etc. running on one phase.

The white "consumer unit" to the right of the MEM switchfuse is a standard single-phase residential-type panel (Wylex brand) tapped off of one phase. It contains just two fuses: 5A for lights & 30A ring for general outlets.

The shop was formerly a key store (as in those places which cut door keys for you) and the 3-phase panel fed the key-cutting machine.

(Ever since "Police Squad" I've been unable to look at one these places without thinking Florida Keys, Francis Scott Keys..... )


[This message has been edited by pauluk (edited 05-30-2003).]

Paul,

On the lower set of pictures, does the movement of the handle actually turn anything off as the outside markings would seem to indicate? Or does it just allow you to open the door?

As the door is opened or closed does it move contactors in and out that disconnect or connect the load?

Bill

Bill,

Yes, the handle does indeed operate a switch. When the door is closed it engages with the mechanism on the lower left. The rod running horizontally across the bottom is turned by the mechanism so that the three U-shaped sections on it bridge the appropriate contacts.

Do you see the porcelain barriers below each fuse holder? The contact on the left of each barrier links to the fuse above it. The contact to the right of each is the lug for the incoming phase. You have to unscrew the brown paxolin cover strip to access the incoming terminals.

The design insures that the parts which remain energized when the cover is open are covered, as far as is possible (415V between phases, remember). The fact that the switch blades rotate out of the contacts on that lower operating rod also provides a visual reassurance that all phases are indeed isolated.

Here's a blown up and lightened picture, where I hope it shows more clearly:




[This message has been edited by pauluk (edited 05-30-2003).]

Ah,

The pic's a bit fuzzy, but my understanding is a bit clearer.

Thanks!
Bill

Now you did it,

I had to come back and add this comment. The combination of this thread and first real sunshine in a week has got me singing in my head.

....I can see clearly now, the rain is gone ....

La La La

Bill

Hi Paul!,
Thanks for the pics.
Regarding that switch-fuse unit, we still use the single phase type over here and I haven't seen a 3-phase type over here for years.
Does the door not have an interlock on it to prevent the door opening with the mains on?
Regarding the first pic of the meter,
what's the bottle of Loctite for?
Is this a new sort of meter sealing technique?.
BTW, what's that small black box on the right side of the meter?.

that pretty neat to see 3 phase service over at uk but here in north american verison is diffrent we carry few diffrent verison most common one are plug in and bolt in and last is current tranformer metering set up but at 400 amp some case 600 amps is the border line for bolt in but for plug in it is restricted to 200 but the current transformer ( ct ) is over 200 amps and above but the powercompany have final say about metering set up for three phase i dont have the picture how the three phase socket look like and main switch device look like here

merci marc

ps i am famiur with french electrical system and it is pretty close what you have it paul

Marc,
Yes, I've seen a variety of 3-phase meters in French houses, and as you say, they bear more resemblance to those used in the U.K. than to the typical plug-in American meter.

Bill,
Sorry about the fuzz....... My camera isn't exactly the highest of resolutions and pictures don't blow-up too well.

Trumpy,
The box to the right of the meter is just a heavy-duty junction box to enable one of the phases to be split to the 3-ph MEM switch and the 1-phase Wylex unit. These boxes generally have two heavy bars in them which allow up to five cables to be joined on each. The second bar is this case has not been used, as the neutral goes just to the single-phase box.

The Loctite? Not sure, but the bottle (and meter top) were covered in quite a layer of dust, so it's been there a while.

By the way, I've also been wearing one of my other hats for this place, as I've written the software to get their operation computerized. They were still putting all bookings down on loose-leaf papers, and assigning and totaling up the jobs manually.




[This message has been edited by pauluk (edited 05-31-2003).]

Just a wee point.Phase conductors should not be brought through separate holes in metalclad switchgear.

lyle -- See https://www.electrical-contractor.net/ubb/Forum5/HTML/000323.html for a current {no pun..} thread.

Lyle,
I think you may have just cracked that nut there, mate.
Paul was asking a question in the "Photos Submitted For Discussion" Forum about this panel and it's wiring method.
I do know that you are allowed to feed singles through a metal panel, but you have to use a Non-ferrous(brass)gland to do this.

(Oops, must have posted at the same time as Bjarney!)

[This message has been edited by Trumpy (edited 06-01-2003).]

Sorry for any confusion guys. I posted a short version of this in the general photos section for better coverage, but I guess that wasn't such a good idea.

No worries Paul,
But what was the actual violation pertaining to this install?.

yep you cannot take single phase conductors through holes on their own,, this would cause eddie currents..

you must either install them with a neutral or earth cable,, or cut a slot between all three holes.

Trumpy,
Yes, the phase conductors entering through three individual holes in a steel enclosure was what I had in mind.

So what do you do for strain relief and cable protection at such an entrypoint?

Is there some sort of grommet materialthat's sold for when you cut out the steel between the holes and turn it into a large slot?

I assume that three-phase setups in the USA have all three phases and neutral running in the same pipe, through a single entry in the panel and then branched inside the panel out to their respective legs on the switch, right?

I'm sorry I ask this silly question, but 3-phase is a bit over my head...

[This message has been edited by SvenNYC (edited 06-10-2003).]

Sven,
If you are installing single cores through a panel, you have to bring them in through a non-ferrous plate or gland.
This prevents Eddy currents from the wires inducing currents in the panel metal.
Over here we generally cut a large slot out of the metal panel and fit an Aluminium plate over this and then fit plastic cable glands into the Al plate.
You're right about the conduit entries.
We seldom use single cores over here, apart from on really BIG jobs (200-400A+ per Phase), it's a lot easier to run Neutral-Screened into a switchboard(it's round) and this is used 9 times out of 10!.
Not a silly question in the slightest.