Electrical question

[Steliz]

Something curious occured and I wondered if anyone on here could offer some logical explanation.

A radial circuit has been tripping in my house and it seems unusual to me. The circuit is for the kitchen sockets which has the fridge freezer as well as a second small freezer, dish washer, microwave, kettle etc. I realise that is a lot for one circuit but when it trips it doesn't trip the RCD or the 16A circuit breaker in the distribution box but it trips the 20A breaker on the incoming phase (I have 3 phase). There are no other circuits on that phase other than the 3 phase boiler.

Why is the overload bypassing the distribution box RCD and local breaker and tripping the larger breaker?

 

[John Brown]

Breakers trip on a combination of current and time duration, as far as I know, so maybe the 20A has a quicker response time.

 

[Mikebart]

If I understand your question,
The kitchen circuit is not tripping but the main incoming is that runs the boiler,
20a breaker should not trip before a 16a breaker unless,
there is a temporary overload on the 16a breaker causing a trip on the 20a breaker due to the response time.
Or a fault on the 20a circuit.
But I am not an electrician so what I have just written could and probably is rubbish.

 

[porker]

If your radial has a load on it that is within the limits of it's break it maybe the additional load on the boiler is sufficient to trip the 20A phase breaker.

The fault could also be a faulty 20A breaker. The issue appears to be an overload issue rather than a residual current leak.

Also agree with the observations of the previous two posters.

Has this just started happening and have you changed anything relating to these circuits like adding any new appliances etc ? Also curious that you say a radial circuit is tripping the phase breaker. Are you saying this happens when you switch on something on the radial? Is the boiler running when you do this?

 

[Spectric]

The RCD will trip only if you have a ground leakage, in other words an imbalance between the current flowing in the live conductor and the return current in the neutral so your problem is just a fault current. What are the two protective devices, the 16 and 20 amp, both the same type apart from current ?

In the UK we have fuses in the plug unless the protective device is supplying a single load such as an oven which requires a 16amp supply so is this the same in Hungary?

You do have a lot of demand on that 20 amp circuit, in the UK you can run a radial to sockets using 4mm and a 32 amp protective device for areas there is a high load. Looking at your loads, the two freezers both have compressors that will have higher inrush current on startup and therefore it is possible that the total load taken with an inrush current exceeds the current rating of protective device. So with the dishwasher running, you turn on the kettle and microwave but at some point the freezer cuts in and at this point it might be where the RCD trips, the reason the 20 amp trips and not the 16 could be down to many reasons but they are very close to ensure diversity.

 

[Steliz]

It's definitely an overload as it has happened when an extra device is used such as the microwave when the dishwasher is running. It has happened 3 times in the last 2 weeks and nothing has been changed although the dishwasher is getting used more than it used to. The boiler provides the central heating so, it is on and off from morning to evening.

I'll check tomorrow whether the breakers are different manufacturers. Can I check if their are any faults on the breakers or is that a job for my electrician?
We don't have fused plugs here in Hungary.

As an aside, or maybe it's related, the boiler is fairly new and was installed about 9 months ago. I've checked the specs and it is 24kW and should be on a 40A breaker however, when the electrician was connecting it up he installed the wrong size cable. According to the spec it should have been 10mm2 but I think he put in 6mm2 (I'll check tomorrow) so he turned the boiler down to be a 12kW unit. Does that sound correct? I did a quick google on cable size requirements for a boiler and there doesn't seem to be a simple consensus

 

[John Brown]

24kw output or input? Is it a heat pump?

 

[cmoops2]

I think Spectric is on the right track - could you tell us what 'type' the 20A breaker is (ie: B, C or D) ? I only ask because your kitchen radial is likely to have more inductive loads than a 'normal' circuit and could (emphasise 'could' as this needs to be estabished by a competent electrician) dictate that the kitchen radial may need to be supplied by a type C breaker ...

 

[Spectric]

when the electrician was connecting it up he installed the wrong size cable. According to the spec it should have been 10mm2 but I think he put in 6mm2 (I'll check tomorrow) so he turned the boiler down to be a 12kW unit. Does that sound correct?

You have paid for a 24Kw boiler yet can only use 12Kw, the 24Kw on three phase will pull around 35 amps per phase as a purely resistive load so why did he proceed with 6mm cable ? Using a 20amp breaker will protect the 6mm cable but it is possible to increase the boiler up from 12Kw to a point where you will overload the cable without the protective device tripping which is not so good.

I did a quick google on cable size requirements for a boiler and there doesn't seem to be a simple consensus

Cable sizing is not just a case of use this size for this appliance, the basic factors are supply voltage, cable length and load along with allowed volt drop and the installation method used.

the 20A breaker on the incoming phase (I have 3 phase). There are no other circuits on that phase other than the 3 phase boiler.

This is not correct under UK electrical regulations, the three phase protective device should only supply a three phase load and none of the outgoing phases should be used for another circuit, that should be on its own single phase protective device. So in the Uk that boiler would be on it's own circuit and fed from it's own RCD. The kitchen would be on more than one radial to both spread the loads and more importantly help ensure that you don't lose power to everything if one rcd trips, especially important for freezers, fire alarms, intruder alarms and heating systems. As an example I came across some years back where someone had been DIY'ing. An outside PIR light had blown and tripped an MCB, unfortunately this MCB also supplied the heating system which gave the owners a shock when they came back from a holiday as pipes had frozen and on thawing taken out several ceilings and gave them an indoor pool.

I would think that going by the loads you have on that radial, only a 16 amp rcd that the design is close to being at its limityou might be borderline and the loop impedance may not be low enough to accept a type C breaker which requires 5 to 10 times rated compared to the type B which is 3 to 5 times rated current.

 

[Steliz]

The breaker types are C25 for the boiler circuit and all the others including the incoming phases are B type. The cable for the boiler circuit has the text 5G4 mm2 on it which suggests it is 4mm2 and not 6mm2.
I'm not in the UK and I know nothing about Hungarian regs but the boiler is on it's own circuit. The kitchen sockets are on one circuit which is also the only circuit in the house using that phase. All other circuits in the house are spread over the other two phases. The RCD is 25A, it is the individual circuits that are 16A.