I have some experience on selectivity in maritime systems. However now, I'm having some issues at home. In the breaker box there is an incoming supply with a mini circuit breaker rated C25. Then there is a lighting loop with a MCB rated C10. (why they used a C characteristic on lighting baffles me). All MCBs are Eaton brand, type xPole PLN4.

From time to time both incoming and lighting loop fail simultaneous. Thus, we loose power to the whole place. Other outgoing MCBs do not trip. I was unable to find a selectivity table between xPole PLN4 MCBs, just MCB against a fuse link. The main question is: isn't the C10 breaker supposed to trip before the C25 breaker?

I did found a generic graph after resorting to google:

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And the accompanying article states:

Type C circuit breaker trips instantly at current surges 5 to 10 times its rated current. its tripping time lies between 0.04 to 5 seconds. As they can tolerate higher surge currents, they are used in commercial applications such as the protection of small motors, transformers, etc.

This tells me, I should have surge currents larger than 5 times In of the C25 breaker (>125A ?). We are not talking overload, but a full on short circuit. Is this correct?.

Sometimes the interval is a few minutes, other times it seems to take days. Currently I have disconnected all light switches and isolated the incoming cables on each of them. There is nothing on this circuit that switches on and off. And yet, the trip happened twice already under this conditions. Since this is a building, a static application (as opposed to ships), I'm not expecting cables to be moving around in their tubes and the problem to be constant. Power was available at all lighting points, so there is no interruption of a cable.

Besides the real problem of the short circuit: should I replace the lighting MCB with a B10 rating to have better selectivity?

Also, I have some doubts of the engineering skills of whoever installed that breaker box. All MCBs are C rated and there is no differential current protection relay! (Isn't that even a law / NEN requirement?)

Extra background

Might not be essential to answering my question, but never know.

This is an old apartment, built in communist times (Romania) and they used aluminum cabling, all in tubes deep in concrete wall and ceiling. All connection boxes I could find and reach I've opened and found all kind of horrors: cables simply taped together, broken / loose wires in connectors, cuts in insulation due to incorrect mounting, spare / unused cables not insulated (blank conductors behind switches). In 7 locations (including the bathroom!) I found this conditions. Repaired as best as I could by putting shrink tubes over damages and using proper connectors. Also replaced lighting armatures not suitable for the bathroom with new ones that are.


Some time ago we noticed some lights to be blinking, resulting in 2 broken light bulbs. I've replaced those. The problems continued. Two weeks ago the first breaker trip occurred, once. Yesterday morning it happened again and I've started my search and repaired all of the above problems and the blinking light issue was solved. The power stayed on and I thought to be in the clear. Until the middle of last night. In all desperation I have disconnected all the light switches, as mentioned above and on the first time I switched on the MCBs, the trip happened in the first minute, and then again after 10 minutes.

I'm afraid we have to start replacing the cabling in the walls and redo the whole breaker box design.

  • Does the main circuit breaker have a "Test" button on it? I.E. is it RCD? Feb 16, 2022 at 17:44
  • Nope they are the simplest type you could find: eaton.com/gb/en-gb/catalog/electrical-circuit-protection/… (updated the question to include this link)
    – Tim
    Feb 16, 2022 at 17:56
  • Until I read "Extra background" I thought you might have a faulty breaker, and putting too much thought into how it SHOULD behave. :). If you have multiple identical breakers can you swap them around? This will help you diagnose the fault as being a bad breaker or a wiring fault. Also if it's tripping reliably in under 10 minutes you should be able to capture some info with a meter or cheap energy monitor. If you find it's a wiring fault and you have conduits buried in concrete, I think you have the right approach: open up boxes, isolate/remove segments til you find the problem(s).
    – jay613
    Feb 16, 2022 at 18:23
  • 2
    I do think your focus on the breaker curve type is unlikely to be productive. If you have no motors etc then AFTER you solve all your other problems you can swap the Cs to Bs and feel a little safer. But if something is popping your C, it will also pop a B so work on that first!
    – jay613
    Feb 16, 2022 at 18:28

1 Answer 1


To answer some of the specific questions:

Should the C10 breaker pop first? It depends on what’s making it pop. In some scenarios, for example a short circuit in a robust section of cable near the breaker box, they should pop together, or the C25 may even pop first. There are faults that would cause the C10 to pop first but apparently, you don’t have one of those!

Do you have a full blown short circuit? That would be the most obvious explanation for the breakers popping together. But not the only one.

Should you replace the Cs with Bs “to have better selectivity”? After you resolve your problems you should replace them for greater safety. If that is what you mean by “better selectivity” .. yes, you want your fire hazard to be eliminated as quickly as possible. If there is no device that requires the slower reaction, you should replace.

Is differential protection required by law? I don’t know the law in Romania. In the USA, it is required in some situations but it varies by state and city and room and purpose and by when your wiring was installed. If you, as it seems, need to do some substantial rewiring, it's a good idea whether it’s the law or not.

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