We have 3 phase power supply in my home which is used to power single phase 240V 50 hz appliances with common neutral. (Diagram in the link at the bottom)

The only protection we have as of now are MCBs down each phase. But we recently faced a big problem.

Few days back, the neutral wire of the distribution line outside our home broke and fell on the ground. This happened probably due to impoper maintenance by company, summer heat and heavy load. Whatever may be the reason, this fault damaged our wasing machine, air condtioners, water filter, laptop charger and other appliences. This was a big setback for us. So, I am trying to figure out

  1. Why this damage happened and
  2. What can be done to prevent this in future? Are there some device which can be installed to protect the home circuit?

I researched a bit around web and it seems this problem is called neutral fault. This decreses the volatage across some loads and increases across others. High voltage can reach up to 380 to 400 V.

Solution I have found so far is to install a RCBO-4P at input from main lines. So I want to know will this solution work? Will a 4 pole residual current circuit breaker with over load protection trip, if the neutral input is disconnected?

I have tried to simulate what happened here.

Thanks a lot.

Also I found one product which match my requirement, Neutral Loss Protection Relay. But I am doubtfull about it's current rating.


Wikipedia says, "To provide some protection with an interrupted neutral, some RCDs and RCBOs are equipped with an auxiliary connection wire that must be connected to the earth busbar of the distribution board. This either enables the device to detect the missing neutral of the supply, causing the device to trip, or provides an alternative supply path for the tripping circuitry, enabling it to continue to function normally in the absence of the supply neutral."


4 Answers 4


Unfortunately not.
An Residual Current Breaker trips only if the sum of the current on all four wires is nonzero. This residual current means there's a leak to earth, possibly through a human, and the breaker trips.

In your case, the loss of neutral meant the supplied voltages became unequal, some too high, but this won't make the total current nonzero.

There may be other devices which can protect you, but the basic RCB won't.

  • \$\begingroup\$ Thank you. I was also thinking on the same line and that's why asked this question. As you say, a BASIC RCB might not work, but I am hoping there must be some other kinds of CB which handle such faults. \$\endgroup\$
    – Bot
    Commented Jun 17, 2015 at 17:59
  • \$\begingroup\$ I found this comprehensive paper about loss of neutral here \$\endgroup\$
    – tomnexus
    Commented Sep 17, 2017 at 18:49

I am looking for the same sort of protection, and so far the best I can do is use an undervoltage/overvoltage relay PER PHASE to neutral such as http://www.acdc.co.za/rhomberg/docs/AP221mon.pdf which then connects to a shunt trip like this http://www.oez.com/uploads/oez/files/ks/3223-Z01-06_EN_PL.pdf which is attached to a breaker for the phase (i.e. before your RCB).

Your relay will connect via the Normally Open part to the shunt trip with a 5 to 10 second delay. If voltage higher or lower than your preset values it will trip the phase. I woud say go for your safe range as 200v to 255v.

Hope this helps.

p.s. there is an option to use a 3 phase unit with one shunt trip to a 3 phase breaker, but if one phase is lost on the supply side, the relay will trip the whole house. Not ideal!

  • \$\begingroup\$ Thank you Pierre. I am sorry, I didn't understand the connections clearly. Can you please draw it (may be here falstad.com/circuit/circuitjs.html ) to better convey what you are thinking. Also shutting down all phases might also work for me. \$\endgroup\$
    – Bot
    Commented Jun 21, 2015 at 12:20

A 4P RCBO/RCCB won't get tripped in the case of broken neutral fault but three DP RCCBs connected in each phase will trip in the said case of neutral fault protecting all downstream single phase equipments/appliances.


While 4P (3P+N) RCBO won't trip in case of floating neutral where the current returns through one of the other phases, a 2P (SP+N) RCBO on each of the three phases could (but probably not a good idea as explained below).

This is because in a 4P RCD the incoming current equals the return current even in case of floating neutral condition where the current returns through another phase instead of neutral. I1 + I2 + I3 + In = 0.

Whereas in a 2P RCD installed for each phase the equation would look like this:

I1 + In = 0
I2 + In = 0
I3 + In = 0

While theoretically this sounds like a plausible solution, I'm not certain of its practical efficacy when faced with neutral breakage upstream of the house distribution panel, say at the neutral leg of transformer. And the biggest case in point is the European Standard EN 50550 whose conditions for a power frequency overvoltage protection (permanent overvoltage protection) device include:

Prohibition on using earth leakage or current differential as operating principles.

Unless you can get one of these Power frequency overvoltage protection (POP) devices (which I couldn't), the solution is to use Voltage Protection Relay (VPR) and shunt trip (or contactor) per phase to disconnect supply on overvoltage.

I wanted both transient overvoltage (TOV) protection and permanent overvoltage (POV) protection, so the wiring diagram below includes Surge Protection Device (SPD - 3P+N) and VPR+contactor for each phase.

Transient and Permanent overvoltage protection wiring diagram

  • In case of transient voltage spikes (lasting about 30 microseconds, very high voltage/current like 6kV/10kA), the SPD should protect everything downstream to it.

  • In case of permanent overvoltage (lasting indefinitely, higher than normal voltages in the range of 400V), SPD would probably fail after approx. 5 seconds (as per its TOV withstand capacity). The VPR and contactor would respond in <300 ms so it should protect everything downstream to it.

  • Contactor used: Schneider Electric A9C20862, datasheet

  • Voltage protection relay (VPR) used: Selec 900VPR-2, datasheet

  • SPD: Any 3P+N SPD would be suitable. Type 1+2 or Type 2 according to your lightning risk, they all have the same wiring characteristics if its 3P+N.

  • 1
    \$\begingroup\$ A neutral break upstream of the RCBOs will still cause over-voltage, even with seperate RCBOs for each phase. The current flowing out to each load through it's RCBO will still match the current returning from the load through that RCBO, through the disconnected neutral point, through another RCBO, through another load, back through the RCBO and finally back to the other phase conductor. \$\endgroup\$ Commented Oct 16, 2023 at 18:38
  • \$\begingroup\$ @PeterGreen Thanks, it makes sense when you put it like that. \$\endgroup\$ Commented Oct 17, 2023 at 19:23

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