Type C MK Breaker still trips with torodial transformes

[1974stephen]

I've just installed three 300vA 12v transformers onto the downstairs lighting circuit. One is switched on its own, and the other two are switched on together via one switch. I've changed the MCB to a type C 6A, but every now and again switching on just the one switch with the two transformers on takes the MCB. Do I need to install these two on separate circuits with a switch for each, or will upgrading the 6A to 10A make any difference? I've looked for type type D MCB but can't find any.

 

[andy]

I've just installed three 300vA 12v transformers onto the downstairs lighting circuit. One is switched on its own, and the other two are switched on together via one switch. I've changed the MCB to a type C 6A, but every now and again switching on just the one switch with the two transformers on takes the MCB. Do I need to install these two on separate circuits with a switch for each, or will upgrading the 6A to 10A make any difference? I've looked for type type D MCB but can't find any.

you can have a max on 1380W of lighting on a 6A MCB. your transformers will take it to 900W. do you have many other lights on at the time?

 

[1974stephen]

The only other lighting on the downstairs circuit are 40w filament lamps (400W in total) and 2 x 35w 12v lamps via a 105vA transformer. These don't need to be on for the MCB to trip. Hall lights are actually on the upstairs circuit!

 

[Steve]

type D exist, but only for very special applications where startup currents are massive, and i'm not too sure if they would make 6A ones anyway.

 

[andy]

type D exist, but only for very special applications where startup currents are massive, and i'm not too sure if they would make 6A ones anyway.

even if they did you need to make sure that you can install one

 

[Big_Spark]

Lets forget Type D breakers OK...they are NOT for this type of problem and they are certainly NOT for domestic installations, regardless of how you have loaded a circuit.

I would suggest that the paired Toroidals are rewired on seperate switches to drop the inrush current when they energise. These two Tx's have a combined rating of 600Va, however the important thing to note is inrush currents on startup...Tx's can have currents up to 10 times, or more, normal loading on energising, and this can play havoc with MCB's, even Type C's, especially if they are cheaper units.

Turning on both Tx's when the third may already be on and other lighting energised is likely sufficient to overload the ability of the MCB to handle the inrush current, so your first solution is to switch both Tx's individually and ensure that they are not turned on simultaneously.

 

[1974stephen]

Thanks for the advice. As it's all now been plastered, decorated, and is fully installed with no chance of fishing a wire down the capping, and also as an extra switch would mean chopping in a twin KO box rather than a single to accommodate the extra switch, I'll either live with it, or go to my plan b:

Stick a time delay relay in the second transformer so that it doesn't switch on until after 2 or 3 seconds. This might appear a bit weird when switching on, but the third tx switches on after a delay due to the involvement of an MK intelligent LV dimmer, so shouldn't be too bad.

 

[ban-all-sheds]

Thanks for the advice. As it's all now been plastered, decorated, and is fully installed with no chance of fishing a wire down the capping,

This is why conduit is such a good idea.

and also as an extra switch would mean chopping in a twin KO box rather than a single to accommodate the extra switch,

A 2-gang switch is still only a single-width accessory.

What size cable have you wired your lighting circuit in? Could you use a 10A breaker?

 

[kendor]

The trouble with the above posts quoting loads in watts and associated current is that they are not taking into account the inductive reactance of the circuit, and these calculations are not taking into account the Apparent power of the circuit, this is why transformers and other devices with windings of sufficient inductance to affect circuits to which they have been designed will have a KVA rating as opposed to a wattage rating.
As you are somewhat stuffed to alter wiring etc you could look at trying to change the inductive reactance of each transformer to bring them more in line with a true wattage rating by use of a P.F. Capacitor across the input terminals of each tranny. you can get suitable ones at the wholesaler like the ones used in flourescent fittings suitable for mains use.
although perfect matching apart from being undesirable will not be practically achieved as without measuring the reactance you will not know what capacitance to use you will find that it should considerably help to solve your problem.
just as an addendum:
you may need to add some resistive component to the circuit as a slugging device such as a ceramic heat disappating resistor but try the capacitors on their own first.

 

[1974stephen]

The majority of the circuit is wired in 1.00mm, but some in 1.5mm. Looking at my on site guide (out of date now), it appears 1.00mm can carry upto 13.5A when in trunking or in a non thermal wall. As such I assume this means I can upgrade to a 10A type C MCB, BUT will this actually solve my problem, as others quite rightly have pointed out it is due to massive inrush current on start up?

PF capacitors - what are these, and what does PF stand for?

 

[Adam_151]

pf = power factor, its the cosine of the angle between the voltage and current wave forms, for a resistive load there is no angle between them, and so pf = 1

watts = volts x amps x pf is the calc for power, people shorten this to volts x amps, this assumes that power factor is 1, which is not always the case.

va = volts x amps is the simple, but correct calculation, but gives va , instead of watts (apparent power instead of actual power)

Inductive loads means the current wave form lags behind the voltage waveform, capacative loads have the opposite effect, so capactors can be used to cancel out the effcet inductors have on the pf

 

[plugwash]

a C10 will probablly solve the problem but watch the earth fault loop impedance requirements.