Single phase ccu trips out 3 phase Board

[Brace56]

:cry_smile: My facial expression with this job I can't figure out.
Hello first post on here and been searching to see if they was a post like this online.
In a church building, there is a 3 phase board that feeds a wylex single phase ccu with a swa cable 3 core but not sure on size, my best bet is it looks like a 25mm. This single phase board is protected by a mcb 50A within the 3 phase board. Now the single phase board is a spilt load ccu. The main breaker for the single phase ccu is 100A, with two rcd rated at 80ma standard on most ccu. looks like a twelve way board. Now here is the problem. From that single phase board are two radial circuits 32A 4MM s.w.a 4 core, and everytime we plug in 3 pin heaters on the two circuits within 30mins or sometimes 1 hour (varies) it trips out the 50a mcb at the 3 phase board, which means it bypasses the 32a mcb on the single phase,main switch and rcd altogether. Whatsup with that.
At first I thought it was overload but did the formula to find that a 32A circuit can take about 7200 watts, 3 heaters on one circuit each about 2kw.So that ruled out overload, I don't think its a short because of the duration the circuit stays on for before it trips. I was hoping it was the way that they used one s.w.a 4 core to wire 2 radial circuits(shared neutral) basically instead of using a 5 core or running two sepaerate cables back to the board its just the one.But still trips 50A breaker.
I don't really know anything else about the circuit like supply type, but could find out if you have any ideas. Discrimination comes to mind about way it doesn't trip the closet mcb(lowest)i.e 32 a breaker. But other than that I just don't know. Please if you have time remember to reply, we need the heating for the homeless, I am a Electrician struggling, asking my brothers and sisters in arms to shed some light. Thanks

 

[Darkwood]

This is along the lines of what i might of posted but this may have thrown the op completely,but typical of these types of installation and agree with all said.

Partially my idea was to give the OP an idea here of how deep the water is hes stood in, seeing a more indepth reply will then allow him to conclude whether hes out of his depth or not, short quick although helpful answers can give OP a over confidence view of his problem where as a deeper explanation 'well in this particular case' will benefit the OP more, if he follows and understands my points then he can use them to rectify his problem, if hes getting confused with it all i believe he needs to seek onsite guidence from someone with more experience if not pass the job on.

 

[spark 68]

OP will need to be experienced in circuit loading and design if upgrading as suggested as replacing 1ph with 3ph will require a full inspection of all circuits on the 1ph board so as to balance them correctly, note for 400 volts that may become present in switching thus needing warning labels and 400v between adjacent points. What at first might seem like a board and cable upgrade may turn out to be alot more, balancing loads can reduce power consumption and running costs and may need to be calculated to justify the powers who be about the initial cost of rectifying someone elses mess, although not knowing the details of the job i suspect a few more heaters in use plus maybe heavy lighting load which should in this situe be balanced over 3 phases.

You are not wrong there, been there and done this on more than one occassion, the worst one I recall was a hairdressing salon, which had a 3phase supply DB, and SP submain/DB (very similar to this scenario), with the amount of additions and bodges that had been carried out over the years, it ended up a full re-wire/re-design, as this was the cheaper option by far.

 

[robsparky1975]

I have been to a callout at a hotel some years ago,3 phase supply,added on over the years by unexperienced people,fire brigade out before me,melted dno fuse as 1 of the phases overloaded and it took years of abuse for it to melt,it can happen if the installtion is not designed,installed,tested and maintained correctly,i wonder when the last time that church had a EICR(periodic inspection in old money)carried out.

 

[jase]

can still keep the single phase board fed of the 3 phase, as long as you uprate the MCB and check the cable can handle the load, as for balancing the load across the phases, well that 1ph board just acts as 1 load on one phase so you would want to check the other loads on that 3ph board

 

[Brace56]

Seriously thank you all, I Understand the problem better now. Will inform the client of option A - 3p to feed 3p board,(Which I can't see them paying for since they not prepared to spend over £500 on materials. Or option B- upgrading the breaker, but first checking the ccc of the armoured cable before changing anything. Thanks again

 

[Darkwood]

can still keep the single phase board fed of the 3 phase, as long as you uprate the MCB and check the cable can handle the load, as for balancing the load across the phases, well that 1ph board just acts as 1 load on one phase so you would want to check the other loads on that 3ph board

If you re-read the OP you should be aware hes guessing the swa is 25mm to 1ph board, im assuming here its not next door to 3ph board so would need to be fused less than 100amps, common rating 80amps possibly suffice, but he mentions 50amp MCB single pole within 3ph board so as most boards have max mcb ratings of either 50amp or 63amp he will still be too small, he mentions it trips approx 20mins after heaters switched on this would equate to about 75amps on time/current graphs so not really an option to upgrade. Its poor design at the least to have a single phase board loaded up that much when 3phase available and bad design as its tripping it own front end mcb, and this situe should be rectified and in light of the above - the best option is 3ph supply and 3ph board this will balance the 3phase loading reduce the electric bills and justify paying the tarriff to have 3 ph, if as you suggest other phases may be loaded up similar then this would mean a phase per area of church on single phase boards, again poor design as if only using one area of church than you phases are heavily out of balance and costly to the customer in unnecessary electric cost.