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

 

[Edd]

I would be tempted to stic a clamp meter on and see what loads being drawn, and where from.. also im asuming where you put ,,, two rcd rated at 80ma,, you ment 80 amp and 30ma? but yer a clamp meter i think would be your best bet see at what current it trips and why straight away..

 

[Marvo]

Maybe poor or loose terminations at the 50Amp MCB causing heat and leading to tripping.

 

[jase]

is that 3 heaters at 2kw each on each circuit if so that would be 6 heaters at 2kw which would pull 52A

 

[robsparky1975]

Hi,You said that it trips the 50amp breaker,so as said above clamp meter and see what is being drawn,even if the 32a dont trip,those circuits may well be fine but what else is being fed and used at the same time as those heaters ie the sub main wylex board that you are referring too,typical type of installation where things have been added on over the years and soon the sub main is overloaded.

 

[Brace56]

Yeah that's what I mean't Edd cheers. Clamp meter, ahhh yes will use that as well should give a clear indication of loads being pulled. Loose connections makes sense could be where their reset the breaker every 20 times it tripped. 3 heaters at 2kw each on each circuit yes. But 6kw on one radial circuit and 6kw on the other radial circuit. How do you arrive at 52A?

 

[robsparky1975]

ohms law 12kw /230v = 52a

 

[jase]

6kw + 6kw +12kw 12000 divide by 230v = 52A so the total load of those 2 circuits is 52A, are they the only circuits on the board? you said the board was fed via a 50mcb, so the total load of those 2 circuits is pulling more than the what the whole board is rated at

 

[Brace56]

Yes i can confirm circuits have been added, so basically if I measured the Amps travelling through the s.w.a from the 3p to 1phase board and say it measures 124 Amps that means either I will have to upgrade the main switch 100A because it always says not to exceed this as the max rating. Or change to a 3phase board, or change the 50A breaker for a 63A breaker to give the single phase board more power to cope with additions circuits

 

[Brace56]

Ahh yes. Those two radials turned on together will pull 52A, 26A EACH.(52) half of what the whole single phase board is rated at 100a main switch right. So that leaves 12 other circuits on the same single phase board with about 50A to share between them, is that what your getting at. But then if it can't handle that much power why not keep out the 32A mcb.

 

[robsparky1975]

I would check the loading firstly,that cable supplying the board maybe at its limit for ccc so without knowing the csa upgrading to a larger cb may not be the ideal option.

 

[robsparky1975]

With all the heaters on its overloading the submain cb.you seem a little confused

 

[jase]

call an electrician,

 

[Brace56]

Yeah a bit, but your clearing it up. Basically those two radials are drawing to much power from the 50A MCB at the three phase board. But what I cant get my head around is why the mcb doesn't trip on overload. unless like stated before the whole board is overloaded when all circuits are up and running.

 

[Darkwood]

Its obvious circuit and loading design have never been calculated here, your best bet here would be to replace the 1ph sub mains with a 3ph board and new 4core armoured off a triple pole 50amp mcb. this as well as balancing your loads over the phases will give you 150amp of 1ph availaility at submans. Loading calcs etc on existing 3ph board will be required to be worked out, who ever expected connecting several heaters + lighting circuits, sockets etc etc in a church on 1ph when 3ph available is a muppet and should stick to houses as i suspect is there background, no offense to the domestic installers but seems this system has been fitted by someone who has little or no experience of large buildings and or 3ph systems.