who likes problem solving ? i've got a hummdinger of a puzzel, All replies welcome !

[stressfull]

hi there, i have a problem with tripping rcd 30 mA ( well 2 tripping rcd's )
Building "A" has mains incomer, 3 phase TNCS, i have to sub-mains to Building "B", of which 1 is ON peak general power and lights etc and 2nd is OFF peak boiler supply, both are 3 phase and a TT system in building "B" earth rod is 47ohms.
the boiler is 3 phase only (no neutral required for elements) i checked with boiler manufactor if it was ok for seperate supply for control wiring of boiler from Building B general power and lights DB ( due to the fact that OFF peak supply drops out at certain times of the day) that was all ok !
Also the boiler can have a booster ON peak supply from power and lighting DB ( incase it is required to heat house or water whilst on OFF peak supply) of witch was obtained through local wholesaler. The booster supply is controlled through contactors and relays, of which will only allow you to boost whilst OFF peak supply is unavaible.

All well and good, OFF peak supply can come in and out through out the day, protected by 30mA rcd. NO TRIPPING as of yet.
On peak supply is all fine, Main rcd is 100mA as required and additional 30mA RCD Just before boiler control contactors. If OFF peak power is not avaible is is possible to boost for ON peak supply, (booster contactors kick in and all is good)
Now for the part i dont understand !!
If the booster is on and over laps the time of when OFF peak supply comes back in, a relay should kick out ON peak supply and let OFF peak supply the boiler.
But (theres always a but ! ) it trips the 2 x 30mA rcd's and takes out the main 100mA rcd.
I have retested and checked all wiring and RCD's but found no fault, even dis connected earth at boiler (under test conditions) to see if an earth leakage, but still trips.and also linked out neutrals in boiler to see if made a difference but no still TRIPS !!!

As the boiler doesnt require a neutral, (only at booster controls and relay's) could it be because i am changing from an unbalaced 3 phase supply ( On peak general power etc) to a balanced 3 phase supply which is causing rcd's to trip ?
There is no way to totally balance power and lighting and wondered if rcd is picking up the in balanced current on change over from DB.

If your still awake and i havent bored you to sleep, any solutions will be greatly appreciated and look forward to your replies.
Many thanx

 

[Guest111]

Re: who likes problem solving ? i've got a hummdinger of a puzzel, All replies welcom

just a very quick thought,are the phases the same way round on both supplies at changeover point ? l1-l1 l2-l2 l3-l3

 

[Knobhead]

Re: who likes problem solving ? i've got a hummdinger of a puzzel, All replies welcom

Are the contactors for “off peak” and “boost” cross interlocked? If not they should be.
I would suspect you’re paralleling two supplies for a moment causing the RCD’s to trip.

 

[stressfull]

Re: who likes problem solving ? i've got a hummdinger of a puzzel, All replies welcom

hi phil, yes i have checked that, L1 is to L1 etc i even checked it back at the mains and via check meter to make sure, as well as voltage check between L1 on peak to (off peak L1) zero volts and L1 on peak to L2 off peak $)) volts which you would expect.

 

[stressfull]

Re: who likes problem solving ? i've got a hummdinger of a puzzel, All replies welcom

yeah out going side of contactors are linked, so you only have one 3 phase cable supply to boiler. contactors are No1 (on peak L1) No2 off peak L1 which No1 is normally open and no2 is normally closed and so forth for other 2 phases and neutral etc. could there be a problem with contactors not opening or closing all at the same time which could trick rcd into thinking there is and earth leakage ?

 

[snowhead]

Re: who likes problem solving ? i've got a hummdinger of a puzzel, All replies welcom

Are the contactors for “off peak” and “boost” cross interlocked? If not they should be.
I would suspect you’re paralleling two supplies for a moment causing the RCD’s to trip.

Another vote for this, you need a clear gap between on and off peak supplies, altho' it doesn't explain the 100ma tripping.

 

[snowhead]

Re: who likes problem solving ? i've got a hummdinger of a puzzel, All replies welcom

could there be a problem with contactors not opening or closing all at the same time which could trick rcd into thinking there is and earth leakage ?

You're quite likely using 2 phases from one supply and 1 phase from the other at the time of changeover.

 

[electric-ian]

Re: who likes problem solving ? i've got a hummdinger of a puzzel, All replies welcom

Sounds like you need a time delay relay on the contactors to make sure one has dropped out before the other energises.

 

[Knobhead]

Re: who likes problem solving ? i've got a hummdinger of a puzzel, All replies welcom

As the RCD’s tripping at the point of change over, the contactors are where I’d start looking. As has been said a small time delay will ensure a clean change over.

A quick sketch of the set up would be a great help in sorting the problem.