2 x Supplies: 1 x Circuit?

[guzzimental]

Hi,

First of all this is my first post so hello to everybody.

I want to start by saying I am not a plant electrician but recently have had cause to look at some drawings that are confusing me somewhat and hope someone here can shed a little light.

The situation is that there is a 240VAC circuit with various devices connected to it, each with their own RCD across the Live. Instead of being a ring, either end of the circuit is connected to a different incoming supply. Each supply has it's own 20A breaker.

Now this has been explained to me that the reason for this wiring is so if one supply fails then the other will feed the circuit (both supplies are always on). My own basic understanding is that this is tosh and also the protection provided would not be 20A since there are two different drains/supplies.

I am ready to stand corrected as everyday is a school day so hopefully an explanation is forthcoming.

Cheers,

William

 

[Knobhead]

If both supplies are on and there is no bus-section switch then the system is incorrect and dangerous. It is quite common to provide two feeds to a panel but they are usually interlocked so that they can’t both be on at the same time or they are separated by a section switch half way along the bus wiring. In the event of say feeder 1 failing there will be a back feed from feeder 2. Now if the supply to the distribution board feeding feeder 1 fails, there will a feed in to the board via the panel.

So the long and short of it is, your right.

These two set up's are OK

View attachment 9157

 

[Specialist]

Could be Interesting for anyone working on it that did'nt know the set-up:shocked:

 

[guzzimental]

@Tony,

Many thanks for your reply.

I'll check on the interlock situation as it is not on these drawings.

@Specialist

Yup it was 'interesting'. When I get to bottom of it all someone may find themselves in a shallow sandy hole out here!


This will be the latest is some 'discussions' including how is it possible to have a Neutral have 240V on it without tripping anything. Apparently it is a faulty device on the load side (same circuit as described before)! Again, any ideas gladly accepted.

Cheers

 

[Knobhead]

You’ve got me curious, where’s “out here”

As to the other problem you need to give more information.

 

[guzzimental]

'Out here' is Qatar where apparently they are sticking to BS regs (ahem).

The situation I found myself with was this:

I had to swap out a small power supply (12V 500mA) that is double insulated.
Switched off the breaker and checked across L and N for a voltage and there was none.

So I undid the Live - all was well.
Undid the feed Neutral to move it out the way - BAM, 240 through the fingers.

...NOT a happy chappy.

Tested that Neutral to Supply Live, 0v. Then tested Neutral to Earth and read 240.

Site electricians reckon something was wired wrong on the load side to make the Neutral hot yet not trip any breakers. Hmm!?

Oh, and I've just been to have a polite informal meeting and enquired about changeover switches etc.. Nope, both are always live!

 

[Specialist]

Not done by a Yank with a big hat & spurs ?. Been told they have some pretty shall we say novel ways of wiring circuits.

 

[guzzimental]

I have some experience of our former Western colony's wiring (it's cr@p but sort of safe) but this was done by a different former colony.

Oh and this wonder of safety is in a petro-chem plant!

...RUN AWAY


.....QUICKLY....



Ahh, and I meant to add. The two ends of the circuit that are being fed from two different supplies have the different ends in different DBs neither of which making reference to it's partner. Ouch.

 

[Engineer54]

Hi,

First of all this is my first post so hello to everybody.

I want to start by saying I am not a plant electrician but recently have had cause to look at some drawings that are confusing me somewhat and hope someone here can shed a little light.

The situation is that there is a 240VAC circuit with various devices connected to it, each with their own RCD across the Live. Instead of being a ring, either end of the circuit is connected to a different incoming supply. Each supply has it's own 20A breaker.

Now this has been explained to me that the reason for this wiring is so if one supply fails then the other will feed the circuit (both supplies are always on). My own basic understanding is that this is tosh and also the protection provided would not be 20A since there are two different drains/supplies.

I am ready to stand corrected as everyday is a school day so hopefully an explanation is forthcoming.

Cheers,

William

You are correct, you never supply a circuit with 2 supply points that are both simultaneously energised!!

Someone has made a complete cock-up here on this distribution system.
Dual supplies are normally provided by an automatic changeover system that ensures that both supplies remain completely separated. This will only be seen at a single point in the distribution system. That point normally being at the main switchboard, which could also incorporate a stand-by Genny, or UPS system as well as the split bus coupled bus bars...

In essence, your local distribution boards dual supplying means, to this circuit (and probably other circuits) is totally wrong, including the protection levels afforded for this circuit which is probably wired in 2.5mm conductors, or at most 4mm.


I can't think of any reason or set-up, where 2 separate supply points would always be energised feeding into a single circuit, if indeed you can actually call it a single circuit!!...

 

[Engineer54]

I've just read now that the 2 supplies originate from different DB's.... This sounds to me like a dual supply system that's been for one reason or another, purposely ''Defeated''!! It has now left parts of that distribution system in potentially dangerous situations.

I would imagine at one time or another, the main breakers for these 2 DBs would have been interlocked and automatic, ie, .... when normal supply DB-1 energised Secondary supply DB-2 would de-energised. ....or along other similar lines

 

[Specialist]

Sounds like a complete & LETHAL lash up to be honest mate, now you've gotta be wondering what the hell else they've screwed up &left just waiting to kill an unwary electrician. Take care.

 

[guzzimental]

This is a brand new install. Brown field build (well sandy wasteland build). And no, this isn't the only area wired like this!

A little more info. The supplies are coming from a Chloride UPS. Big ****ker, has a whole building to itself. Now, I have been told the two different supplies come from the same UPS but on different busbars.

If each busbar is being maintained separately by the UPS, then is this the same as it just being two separate UPS'? If so then I was under the impression this was a major no-no and apt to result in the fire-services being called.

More fun and frolics after any suggestions to the above

 

[Engineer54]

This is a brand new install. Brown field build (well sandy wasteland build). And no, this isn't the only area wired like this!

A little more info. The supplies are coming from a Chloride UPS. Big ****ker, has a whole building to itself. Now, I have been told the two different supplies come from the same UPS but on different busbars.

If each busbar is being maintained separately by the UPS, then is this the same as it just being two separate UPS'? If so then I was under the impression this was a major no-no and apt to result in the fire-services being called.

More fun and frolics after any suggestions to the above

You are in fact, shorting out the 2 separate UPS bus bars downstream with this arrangement which is bloody crazy!!!, Apart from making life extremely hazardous/dangerous for electricians and lay men alike!!

Apart from anything else, all those protective devices are all null & void!! Or at best the fault protection on all affected circuits has been doubled!! I wonder if they took the final circuit wiring sizing into account, when they decided to dual feed ?? lol!!

These big prime power UPS systems are usually configured with two supply sections, so that if one section of the UPS breaks down it can couple that section of bus bar feed to the working section. They generally have a sophisticated load shedding and by-pass systems incorporated too.

Again, ...i can only think that the site power security system that this UPS was originally designed for and probably wired for too, has been deliberately defeated and now this UPS is feeding both normal and stand-by DB's as it's normal duty. The problem being, that the distributed site supplies remain wired/connected for a normal and stand-by supply!!!
Maybe, this UPS (despite it's physical size) can't meet the sites needs without using Both maintained outgoing bus bar sections... That's the only reason i can come up with, with the information given to date!! ...lol

 

[Knobhead]

With 20A protection on each incoming supply the link between the UPS systems isn’t going to last long.

[h=1]But it all comes down to the fact the installation is downright dangerous[/h]
Key interlocked feeds YES
Auto changeover YES
Permanent twin feeds NO

Bill, your going to have to make a stand on this before someone is killed, it’s already bitten you once.

 

[Engineer54]

With 20A protection on each incoming supply the link between the UPS systems isn’t going to last long.

But it all comes down to the fact the installation is downright dangerous


Key interlocked feeds YES
Auto changeover YES
Permanent twin feeds NO

Bill, your going to have to make a stand on this before someone is killed, it’s already bitten you once.

OR ..... Get rid of the dual feeds downstream.

Never seen this arrangement before anyway. Any dual feeding could be done conventionally at the main switchboard. No idea why they would do this downstream, it creates absolute havoc when anything goes wrong and renders the distribution system virtually sterile to any future updates/additions etc!!

Just wondering what sort of testing and commissioning was carried out on this installation prior to energising?? Surely that would have shown them that this system was a duffer!! ..lol!!!