Phase to Phase Short

If the cable is large enough, yes. If not, then the cable may melt, with conflagrating consequences.

Why any Electrican would do this is completely abstruse.

PWSlack thanks, i'll elaborate little further main incomer is 63A TP isolator than branch breakers are of 10Amps & 20Amps, B & R phase are lighting circuit through 10Amps SP breaker, conductor is 2.5 Sqmm single core. will these outgoing breakers trip on phase to phase fault?

Is the installation completed and certified to BS7671?

Bear in mind that in order for this circumstance to arise, the simultaneous failure of the insulation of two conductors would need to happen...

yes, but its only a possibility.

Well, either the certification document exists or it doesn't; it cannot be a <...possibility...>!

Possibility In case of insulation failure, grouped circuits traveling in a GI trunking or any other means of raceways.

DOES THE EFFING CERTIFICATE EXIST?

PWSlack thanks for the feedback.

Not done yet. If the certificate exists then there is no need to worry. So does it exist or not? Has the installation been designed, installed, inspected, tested and certified to BS7671 or not? Please answer either yes or no!

This is becoming tiresome.

PWSlack thanks for the reply, the attached picture is just an imaginary to make the theory more clear,as of standards BS regulation all complied. my question please don't misunderstand, i just want to make clear if by chance any two phases come in contact in a circuit feeding different loads, by insulation breakdown etc. likely may/may not be in the DB but somewhere in the trunking/raceways etc. outgoing MCB's are Single Pole.

In that case, the Original Poster needs to try it using the installation to hand. That all other CR4 readers are standing well back when it happens is highly recommended.

Perhaps it has already happened. It might explain the lack of response to simple yes/no questions.

<unsubscribes in despair and takes cover well away from that dis board>

"If the two circuits come from the same phase or leg of the bus..." - surely the OP has made it clear that R and B refer to different phases (from the thread title if nothing else).

As Peter says, it's probable only one MCB will trip on a phase to phase fault. Once one has opened there's nothing for the other to do.

Some minutes are apparently not learning curves.

Tornado thanks.

Trust your gut feeling on this: if it does not look right it is not right!

Joining blue and red without safety switching device seems sooo wrong that it probably is.

But thats just me and I am not an electrician.

It is our owner to know about this. Thank you very much.

thats not a wire anymore, its a fuse, it will melt quickly

Yes. The phase breaker(s) (MCB not isolators) will trip provided the wires don't burn out earlier. One thing is sure. You are going to see some fireworks. Please keep a fire extinguisher handy. Is this an attempt at revalidating proven electrical concepts?

Pattabhiraman thanks for the answer.

Hell bells man.. don't tell him to use a fire putter outer..... I doubt if he'll know which type to use!

I understand that this is a theoretical question from somebody associated but not responsible for the safe design of this power distribution. Obtaining proper inspection certification for a power distribution is very important and should always be done. A certified design will mitigate this type of a fault but does not answer the question of how the displayed device will respond to such a fault.

Assuming that the circuit breaker in question is the pink handle device and that the short circuit path can maintain the fault current long enough for a breaker to trip then the answer is yes. A phase to phase short should trip all phases of this breaker. This breaker is a four line ganged circuit breaker. You'll notice that all four toggles are mechanically linked together. Moving one toggle moves all four. I am assuming from the image resolution I can see that the mechanical linking is a permanent linking and not a jury rigged configuration. There are circuit breakers out there that look like they can be ganged together with added hardware but a tripped breaker does not generate enough force from one toggle to move another toggle.

This is why proper certification requires an inspection and a design review. It is so easy to just look like it should work.

redfred thanks for the reply, the breakers are single phase 10A SP conductor 2.5 Sqmm (the above graphic is an imaginary to make the question more clear), its a rare case but if its going to happen than will the breaker's going to protect?

There's no magic to this, a phase to phase to fault has a short circuit current that is approximately 87% that of a three phase fault! Which means that a lot of current is going to flow, limited only by the impedance of the supply grid, stepdown transformer, and the supply wires.

It is simply a race as to which breaker will act first. And OP had better be sure that the asymetrical interrupting current of his entire panel is up to the task as suggested by others, or the picture in post #24 will not be to far from the result.

Check out page 5 (196) of this link for the calculations. If you have to ask this question you are not qualified to take money from your client to answer it.

RAMConsult thanks for the answer

either breaker R or B will trip. or the main incoming breaker will trip, or the main fuses will blow or the fire brigade will cut the power to stop the fire. Is it me or are questions these days getting..... what is that word I'm looking for, on the tip of my tongue..

Assuming the devices have been selected to meet the expected fault level of the system, then yes they will trip, almost certainly both would go. It's very unlikely that a 2.5mm cable would fuse in the time it takes a 10ka MCB to clear the fault, if it did, then the circuit is not compliant anyway. While you have drawn it strangely, it's really no different to a phase-phase fault in a 3phase cable /device when someone decides to cut through it with a spade/digger

LAA_Lucke thanks, this explanation is very clear.

LAA_Lucke thanks for clearing the misconception regarding the question, i wonder how LAA_Lucke clearly understood the above question, but some culd't, was that misleading one, i do really apologize for my bad english.

nothing wrong with your English, some people have lost the instructions for their Mark 1 eyeballs so they don't read right!

Since the "B" & "R" circuit branch were only derived from the same phase, protected by "X" phase circuit breaker, this will appear to be a simple 2 circuit breakers that has been put in parallel by an accidental or intentional short! Ideally the 'B', 'R', and 'X' breakers will and should not trip! This condition holds true Only on the condition that all circuit components in this part of the DB are characteristically and electrically identical, balanced with no other loads connected!

But when "B" & "R" circuits are fully wired, terminated with corresponding loads, at a minimum, one of the 'B' & "R' circuit breakers will trip! Due to the imbalance in current brought about by the difference in each circuit branch impedances which is physically impossible to balance! The "X" breaker and other, if part of a multiphase circuits may also trip as phase protection comes into play!

Wrong! Please explain how you worked out R & B are the same phase?

"in the attached pic where "B" & "R" phase coming from a single phase breaker"

Seeing as we are using BOLD WORDS...

You said...Since the "B" & "R" circuit branch were only derived from the same phase, protected by "X" phase circuit breaker, this will appear to be a simple 2 circuit breakers that has been put in parallel by an accidental or intentional short! Ideally the 'B', 'R', and 'X' breakers will and should not trip!

R is on one phase and B is on the other phase. How can that NOT be a short?

Have you NEVER worked with 3phase power supplies before?

Read post 2. It says R & B phases, not breakers

It's my fault to just read the 1st post as posted by the OP! I did not bother going thru all of the postings. I'm sorry its a mistake on my part!

vsar thanks for the reply, "B" & "R" is blue & red phase through a circuit breaker of 10A single pole (original post i mentioned it as single phase breaker). "X" is a short circuit point either in the DB itself or somewhere in the trays/trunking.

You will see that they come from single phase breakers as opposed to 3 phase. The are connected to the black & brown phases. But the question I believe is hypothetical and not based on a cable joining the two directly

I would suggest its fairly clear to be 2 phases, almost certainly meant to be blue & red

Edit: in fact it says the incomer is TP

Speechless!