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Monday, 24 February 2014

The pain of a Neutral to Earth fault



If you are not sure what an earth leakage fault is or how the residual current devices detect it such as an ELCB, RCCB, RCBO or RCD then please look at this post where I explain what these terms are and what it all actually means.  In this post I will look at an obscure fault, which can be perplexing and describe what it is and what can cause it.



Please note that this article is my opinion and cannot be considered advice.  For any issues regarding any electrical fault or installation, seek the advice and knowledge of a qualified electrician fully conversant with the laws and regulations of your country or region. Where the applicable regulation specifies a specific installation or action, this must be followed to comply with your regulations.





Index.


  1. What exactly is an, "Earth" in a domestic situation?

  2. What is an equipotentially bonded area or zone?

  3. Do RCDs have significant limitations?

  4. Aren't earth leakage faults simple to diagnose?

  5. So what is a Neutral Earth fault?

  6. Wouldn't an Earth to Neutral fault blow the fuse or trip the MCB?

  7. With a neutral earth fault, the identification process can be much more difficult.






What exactly is an, "Earth" in a domestic situation?




In the UK the earthing inside a domestic situation is called, "equipotential bonding."

  • equi - meaning the same
  • potential- meaning voltage
  • bonding - meaning connected together

 So "equipotential bonding" means "connected together so all is at the same voltage"

Essentially, anything such as fixtures and fittings, that are accessible by humans or pets which could potentially carry a current, are bonded electrically. Appliances, with conductive cases, are usually connected via the provided earthing on the plug socket which they are attached.



What is an equipotentially bonded area or zone?



The idea is that if something conductive picks up a current, the current will flow through the earthing equipotental bonding and either pull out the trip or dissipate harmlessly outside of the bonded area.  The result is that you shouldn't have a situation where you could hold two separate conductive objects, which are at different voltages and pose a shock hazard.

  • Situations such as; turning on a tap or taps, leaning against a radiator, brushing against an appliance such as an amplifier, etc.  

  • The bonding of pipes is essential where they are metal.  Non-conducting joints need to be bridged electrically, so maintaining a contiguous electrical route along the pipe's path. 

    • Special emphasis is made where the pipe enters the property or bonded area. Often a heavy gauge earth wire is connected from these locations to a central earth point or busbar. As modern materials such as PE (polyethylene) are being used for pipework, it is not clear how this affects the equipotential bonding regime. (Please check rules applicable to your country for confirmation as they are different).

    • supplemental bonding is used between pipework and at significant areas such as at the water heater, water tank or tap connections. This allows the convenient connection of different services such as gas, central heating feed and return, hot and cold water feeds, etc. It also stops potentially dangerous voltage differences occurring in these locations if there is a failure in another part of the bonding regime.

  • If a metal tap is fitted to plastic feeder pipework in a plastic or ceramic sanitary ware and the hot and cold taps are a single unit, then it does not usually need to be part of the equipotential bonded area. (Please check rules applicable to your country for confirmation as they are different) .

In places where the mains are supplied as two individual phases, the neutral conductor may be derived electrically and an earthing spike is used as a dissipation point.  Within the equipotential bonding area, everything is connected electrically and referenced to the earthing spike with its derived neutral terminal. In certain countries such as Australia the reinforced mesh grid of the foundations also need to be included in the bonding network regime.


  • In reality and in open country, the physical earth you stand on may have significant voltage differences over small areas.  This variation is entirely natural and can be caused by underground conductors, geological processes and atmospheric conditions, the most notable being during lightening storms. The equipotential bonded area is supposed to isolate the consumer from these fluctuations and provide an electrically safe and uniform environment.

You are at the will of the gods outside the equipotential bonding area, as you could come across another equipotential bonded area using a different phase neutral. Consequently they could be at totally different voltages and thus represent a shock hazard.
It is dangerous to assume that the earth voltage is the same between domestic units being in different and separate equipotential zones. People have been killed making that assumption. Helping to mitigate this risk is really the territory of the Residual Current Device or RCD. Note for the best protection both parties need to have RCD protection.

  • Note if portable power generators are used within a domestic environment, be it emergency or social, It is essential for the generators to be grounded to the same equipotential earthing points. This is to avoid potentially lethal ground/earth voltage differences. The only caveat is if the generator and equipment is totally isolated but it would still be wise to use an RCD as additional protection.




Do RCDs have significant limitations?



Yes, the RCD is not connected to the earth lead and so cannot detect earth problems directly, for example:

  1. It cannot detect a voltage difference between earths on different circuits.
  2. It will not detect any current within the equipotentially bonded area or between zones.
  3. It cannot detect a voltage or current in the earth conductors within your equipotential zone.
  4. It cannot detect if the earth connection at the consumer unit has failed or has been disconnected.
  5. It cannot detect if the earths are properly bonded within the equipotential zone.
  6. It cannot protect from residual leakage where the mains or power is isolated from the main circuit, such as from the secondary winding in a transformer.

    • In addition an RCD cannot protect from many other problems, for example:

  7. It cannot detect a low resistance or short circuit between live and neutral.
  8. It cannot protect for a reversed live neutral connection due to incorrect installation.
  9. It cannot protect against overheating due to the arcing of badly connected terminals.




Aren't earth leakage faults simple to diagnose?



Sometimes.  Most of the time the "Live" wire is responsible for a residual leakage fault. This is predominantly due to the higher voltage differential and so the greater potential to drive a current where it is not supposed to go.  Live to earth faults are easy to diagnose at the appliance by just switching off the socket feeding that appliance.  If the appliance is permanently connected to the circuit or fault is in the circuit itself, then manually tripping the MCB is usually sufficient to find which circuit is responsible.

Sadly this method may not be sufficient to diagnose an earth to neutral fault.





So what is a Neutral Earth fault?



A neutral to earth fault means that the neutral conductor side of the circuit is leaking the current rather than the live side. In most domestic applications, the earth and the neutral are usually connected together meaning that the voltage between them is small.  It may be small but not insignificant!

  • Normally, no current is flowing in the earth line, so the voltage along the circuit will be identical throughout, and exactly the same as in the neutral wire where it connects to the earth point. The purpose of the earth bonding is to ensure that this is always the case and is known as equipotential bonding.


The fundamental physics of electrical systems means that if there is any appliance consuming power within a circuit then the current throughout the circuit will be identical when measured at any point within the circuit.

  • If there is a current then there is, "always," a voltage drop!

The resistance in the neutral conductor should be small but it cannot be zero unless it is a superconductor.  Thus the more current flowing the larger the voltage drop in the neutral line between the appliance and the consumer unit. With a voltage drop, it means that there is a potential for the current to leak out to earth, even though they are electrically connected together.

  • It is important to understand this concept as it can lead to a situation where a short between earth and neutral in one circuit may not be sufficient to trigger the earth leakage trip (residual current below 30mA). This is because the voltage difference is too small to generate sufficient current flow to trip the RCD.

  • It is possible that high current flow in a totally different circuit may raise the overall neutral line voltage sufficiently to allow the residual current to then trip in the faulty circuit.  Thus, it can appear that the RCD trip is intermittent and the wrong appliance or circuit is implicated as the cause.

    • For a description of such an event please see the story in my post titled: Finding a neutral earth fault called, "The night light mare."



So a Neutral to Earth Leakage fault is when a fault develops and the neutral and earth are connected together sufficiently to cause a current flow out of the neutral conductor and into the earth (or visa versa).

  • In a circuit protected by an RCD ,such as an RCCB or RCBO, the circuit fault conditions causes the RCD to trip.
  • In a circuit unprotected by an RCD, it will appear to function normally even though the current is not balanced between the live and neutral. 

  • In neither the of the above cases would the circuit's overcurrent protective fuse be pulled out. As the RCBO effectively is a combined overcurrent fuse (MCB) and RCD, it will trip but not due to an overcurrent condition.



Wouldn't an Earth to Neutral fault blow the fuse or trip the MCB?



No, the neutral and earth conductors are normally connected together before the consumer unit anyway, so the, "load," current flow would appear normal to the overcurrent device. In this fault state, the current can flow through both the earth and neutral and still return to the neutral line at the combined earth bonded connection point.

The voltage difference between the Neutral and Earth conductors varies from nothing to small and so there is no overall significant current difference whichever path it takes.  In the case where the neutral is damaged and the return path for the current is solely via the earth conductor, there still wouldn't be insufficient difference in current draw to pull the fuse or even indicate a fault condition to the user.

This type of imbalance would only be detected by an RCD such as an RCCB or RCBO being triggered.

  •  For a description of such an event please see the story within my post titled: Finding a neutral earth fault, called, "The screwed up mains circuit."





With a neutral earth fault, the identification process can be much more difficult.



If the appliance is at fault, then switching off the appliance at the socket may not be sufficient to isolate the appliance from the neutral conductor.  The reason for this inconsistency is that sockets and switches can be either dual pole or single pole. A dual pole switch will disconnect the live conductor and the neutral conductor but still leave the earth conductor intact and engaged(assuming there is an earth). A single pole mains socket will only disconnect the live conductor but leave the neutral and earth conductors intact.  So a single pole switch will not sufficiently isolate an appliance to allow simple fault diagnosis!


The diagnostic approach is simply a system of elimination by physical disconnection.

  • Single pole switches are cheaper than dual pole and so are relatively common.  From the outside there is no easy way to tell them apart.  The best option is to physically remove the appliance plug from the socket ensuring an identifiable physical disconnection.

  • Similarly, if the problem is in the supply circuit then isolation cannot be as simple as tripping the MCB because the MCB only switches live.  Equally if you have a circuit with a single pole RCBO you have the same problem as with the single pole MCB.  Essentially you need to manually disconnect the Neutral wires for each circuit being diagnosed. 

Once a likely candidate, or candidates, have been found it is important to confirm they are actually faulty.  This may be best performed by a qualified electrician with the appropriate test equipment.

  • As described above, a current flow in the neutral circuit will cause a change in voltage between the earth and neutral for all circuits.  This voltage difference can get to a level where the RCD detects a sufficient imbalance in current to trip the circuit. So a fault on one circuit may be caused by a large current flow on another circuit. This situation is likely to lead to a false diagnosis.
  • To make it more obscure, the high current and circuit causing the change in the neutral voltage may not even be protected by the same RCD.  It is entirely normal for a small voltage difference between neutral and earth caused by high current flow.  In the UK there is a regulation specifying the maximum allowable resistance of any one circuit to keep this voltage difference within a specified tolerance.
  • Thus it is important to confirm that the suspected circuit is confirmed as the reason for the fault.  Failure to do this will lead to a situation where the trip is blamed for being over sensitive or intermittent when in actual fact the fault was incorrectly diagnosed.

Once the equipment or circuit is confirmed as the reason for the neutral earth fault it can be dealt with.  Professional electricians will often have access to test equipment such as a Portable Appliance Tester (PAT) which should be able to confirm that an appliance is faulty or not.

It is very annoying to discover you consigned something perfectly serviceable to junk by misdiagnosis.

In my following post will describe some real world issues I have come across regarding Neutral to Earth faults.








Please note that this article is my opinion and cannot be considered advice.  For any issues regarding any electrical fault or installation, seek the advice and knowledge of a qualified electrician fully conversant with the laws and regulations of your country or region. Where the applicable regulation specifies a specific installation or action, this must be followed to comply with your regulations.




For further reading and references:  Please see my resource page




1 comment:

  1. Thank you for this posting. I live in a place with no qualified electrician available, and have been experiencing exactly the sort of infuriatingly mysterious problem you describe. The act of switching on a bank of LED room lights (high inrush current?) or operating a microwave oven has been tripping the RCCB on a different group of circuits entirely. Swapped the RCCB for another - same result. Even with all the MCBs for that tripped group turned off, and all the wall outlets in that group unplugged, the RCCB would still trip. Only when I disconnected entirely one of the neutrals from that group was the problem resolved. Now I assume there may a leak to earth somewhere on that neutral line. Anyway thanks for explaining a difficult topic clearly to laypeople...

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