Earth Leakage Breaker (RCD) Tripping

[AndyD]

We get a lot of threads about earth leakage faults on the forum so I thought I’d make a thread outlining in laymen’s terms what an earth leakage breaker is, what it does, why it trips and how to go about alleviating some of the problems that can be caused when it does.

The info here is aimed at homeowners and domestic users who have a basic DIY-level understanding of electrics. I’ve attempted to avoid complex technical terms and explanations in order to keep it accessible to non-professionals.


What is an Earth Leakage Breaker?

The earth leakage breaker is officially known as a Residual Current Device (RCD) and in the interests of my one-fingered typing I’m going to refer to it as an RCD from now on.

The RCD is a device that is designed to disconnect the supply to a circuit or several circuits or even the entire house installation when a hazardous fault known as an earth leakage fault occurs. It’s identifiable as a wider than normal circuit breaker, often with a double lever on the front and it also has a test button which should be pressed regularly by the home owner to ensure it operates correctly.

RCD’s come in a wide range of types, some have time delays and they come rated to operate at different leakage currents, some detect DC leakage faults as well as AC leakage and they also come as 3-phase and single phase configurations. To keep things simple I’m only covering the general purpose 30mA single phase RCD which is widely used in standard domestic electrical installations.



Various examples of earth leakage circuit breakers (RCD’s)

[AndyD]

What Are Earth Leakage Faults?

Current flows through a circuit the same way water flows through pipework. With water, if 10 litres flows into one end of a pipe then 10 litres will flow out of the other end. Similarly with electricity if there’s 10Amps flowing into one side of a circuit (Live) there should be 10Amps flowing out of the other side (Neutral). Current in always equals current out.

The thing that keeps the current in the circuit is insulation. The only time current is ‘lost’ or ‘leaks’ from a circuit is when the insulation has failed. Sometimes insulation failure is spectacular and results in a flash and a bang, when this happens sufficient current ‘leaks’ to cause a normal circuit breaker to trip. Sometimes however insulation failure is more subtle and results in only a slight loss of current from the circuit, this is what the RCD is designed to detect and without an RCD the circuit would remain live in the case of this kind of fault.

You might wonder why we’d be worried if only a few thousandths of an Amp was being lost but a few thousandths of an Amp is all it takes to kill someone if it flows through their body, hence the great importance we place on even small amounts of leakage.

Most people have experienced a tingling feeling of a small electric shock at some stage, this tingling is caused by only 1 or 2 thousandths of an Amp flowing through them and out through their feet into the ground (earth) they’re standing on. When the current flowing through someone is greater than 5 thousandths of an Amp (5 milliamps) you get muscle reaction that causes cramp, 10-20mA will give you a violent shock and 30mA is considered the maximum shock that shouldn’t cause death. More than 30mA of current flowing through a human body causes interruption of normal heart rhythm and can lead to death.

[AndyD]

What does an RCD do?

The RCD in a domestic DB is designed to trip almost instantly when 30mA of current is being lost from a circuit which would be a 30mA earth leakage fault. Not only must it trip when there’s a leakage fault, it must disconnect the circuit within milliseconds and it must disconnect both the live and the neutral legs of the circuit.

To detect such small currents an RCD is almost a thousand times more sensitive than a normal circuit breaker that protects a socket circuit because the RCD is there primarily to protect people from shocks and not to protect the circuit itself or the appliance from overload. The RCD constantly monitors the current flowing through the live and the neutral sides of the circuits because any difference in these currents is caused by earth leakage.

For example if there’s 10 Amps flowing on the live side of a circuit and only 9.95 Amps flowing out on the neutral side then 50mA is being lost from the circuit and the RCD will trip.

Some RCD’s offer a combination of overload and earth leakage protection in the same device but many are not designed to give overload protection and there’s often some confusion surrounding this. If you look at an RCD it usually states 60A on the front but this doesn’t always mean it will trip when the load current exceeds 60A it just means its internal contacts can only disconnect a 60Amp load if there’s an earth leakage fault.

[AndyD]

Why does the RCD cause so much inconvenience?

Off the bat I'll say there are some very poor quality earth leakage breakers on the market. Many of the cheaper types are very prone to nuisance tripping or not functioning as they should when there’s a fault. In my opinion it's definitely worth paying the extra for a brand name like Schneider, Merlin Gerin, ABB, or CBI depending on the type of mounting rail in the DB.

If an earth leakage breaker (RCD) trips intermittently, from my experience, 98% of the time there's a fault or a number of faults on the circuits it supplies that's causing the tripping. You do get the very occasional RCD that is too sensitive by a significant margin but it's the exception not the norm.

The RCD is unlike most other circuit breakers in your DB because it monitors both the live and the neutral. Just switching off an appliance using its built-in switch or the switch at the socket or manually tripping a normal circuit breaker to disconnect a circuit doesn’t work if you're trying to find an earth leakage fault because the neutral is still connected and the RCD will still ‘see’ the fault. To effectively clear (remove) an earth leakage fault appliances need to be completely unplugged and circuits need to be completely disconnected on both the live and neutral side. This makes finding earth leakage faults difficult and confusing for homeowners.

In most houses nowadays there’s electronics everywhere and every house or business has dozens of electronic power supplies (switch mode supplies). A few examples would be computers, printers, laptops, phone chargers, modems, routers, hi-fi, TV’s, X-Box, M-Net decoder......the list is endless. The problem they cause is that even under normal operating conditions they have a small amount of leakage current. It's generally one or two milliamps and we call it 'functional leakage' to differentiate it as being acceptable, unlike earth leakage fault currents which aren't. The RCD unfortunately doesn't know this leakage current has a cute name and isn't considered a fault current so when you've got possibly a dozen of these things in your average home the RCD sees all these small leakage currents as faults. Realistically they could add up to 10-15mA which means the 30mA RCD is effectively now only allowing 15-20mA of earth leakage fault current before it trips. Add to this a few LED lighting drivers, electronic ballasts in fluorescent fittings and noise/surge suppressors in your Hi-Fi that also have standing (functional) leakage to earth and next thing you know you've got nuisance tripping even though there's actually no actual 'fault'.

[AndyD]

How Can The Nuisance Factor Be Reduced?

To avoid nuisance tripping the layout of domestic electrical installations needs to change. RCD’s used to be very expensive and traditionally in South Africa the RCD was used as a main breaker and monitors the entire installation so only one per installation was required. This means that as soon as the combined earth leakage fault currents and functional leakage currents for the entire installation exceeds 30mA it will trip even though no single circuit has a large enough fault that would normally warrant its disconnection. Nowadays, because of the already considerable and ever increasing 'acceptable functional leakage' that comes with electronic power supplies, a single upfront RCD should be considered poor design because the inconvenience factor is high but unfortunately it’s the cheapest way of doing things and still being compliant with the existing electrical regulations it's still common practice.

A better option is a split DB with two RCD’s. This means that there’s less inconvenience when there’s a trip and it also goes some way to accommodating the acceptable standing leakage currents as outlined above. The latest development is that every single circuit breaker is also an RCD, these circuit breakers are called RCBO’s. This system is far more preferable, it means that an earth leakage fault on any circuit will trip only that particular circuit. Obviously this system costs considerably more money but prices are still coming down and already they’re within the budget of many owners of medium to large houses.


Tripping Faults

As far as the homeowner is concerned there’s two main types of RCD tripping fault. There’s the type of fault that’s occurred out of the blue where the RCD has tripped and point blank refuses to reset and there’s the type of fault where the RCD trips once in a while and resets quite easily until a few hours/days/weeks later when it trips again.

Before we go any further, from experience, if an RCD is tripping there’s a 98% probability that it’s caused by a fault or a number of faults on the circuits it’s supplying rather than it being a failure of, or problem with the RCD itself. In other words, if you have an RCD that’s tripping you should approach it as being a fault somewhere on the electrical installation or in one of the items that’s plugged in, not as it being a faulty RCD.

[AndyD]

What You Should Be Doing

Regardless of the type of tripping fault you have there are some things you should do as a matter of course and if they’re done properly you stand a good chance of finding the problem.

1. Check all trailing cables. Appliance cables are particularly prone to wear and tear, abuse and damage. Go to every appliance and unplug it, then run your hands up and down the length of the cable and visually check the plug for any signs of strain damage. Don’t limit this procedure to the items in the house, go outside and check the pool pump and the gate motor and any other things you can think of. Get up in the loft as well and visually inspect the cabling for rodent damage etc.

2. Check for damp patches and water leaks. Moisture is a big cause of earth leakage faults, especially intermittent ones so again, be thorough and methodical. Check for garden lights getting soaked by the irrigation. Check under the fridge and washing machine and around the pool pump for water or signs of damp. Check for roof leaks that may be causing cabling or lighting to get wet. Often, fixing a water problem will also fix an earth leakage fault.

3. Check for general damage. Seemingly inconsequential things can cause earth leakage faults. External sockets with missing box lids, external light fittings that are badly corroded and even a broken gland where there’s a cable entry. Take a slow walk around and check the lights for internal condensation and generally use your eyes.

If I arrive at a home with an intermittent RCD tripping fault this is exactly what I do first, I’ll spend at least 20 minutes to check for the obvious before looking for the trickier hidden faults.

Another thing I’d be looking for is poor quality workmanship. Electrical work that’s been done by the homeowner or someone who’s not an experienced sparky is far more likely to be the cause of faults. There’s a way of doing things and dozens of little tricks that an experienced installer uses and they prevent future problems. It’s a lot of little things like using the correct glands for the particular cable, leaving drip loops at outside light fittings and so on but the difference is a reliable installation verses one that may or may not be.

[AndyD]

Point Blank Refuses To Reset

Let’s deal first with the type of fault where it just won’t reset no matter how much you curse at it. The chances are that this would be caused by a single earth leakage fault that’s quite large. The fault could be on the installation cabling or it could be on something connected to the circuits that are affected.

Now is a good time to have a think about anything that may have changed recently. For example have bought any new items and recently plugged them in? If so focus on the things that changed recently and undo those changes by unplugging the new appliance and seeing if the RCD resets with it disconnected.

Next step if the fault persists is to unplug every single item and appliance on the circuits that are without power due to the tripping fault then try again to reset the RCD.

If it resets then you should be able to figure out which appliance is the problem child by plugging them back in and noting which one causes the RCD to trip again. If it doesn’t reset then it’s time to call in your favorite electrician because the fault is somewhere on one of the actual circuits so you’ve done everything you can without electrical test equipment.


Trips Once Every So Often

The intermittent type tripping fault can be a lot trickier to find. There’s a high likelihood that an intermittent trip is caused by a combination of acceptable functional leakage as well as a number of small earth leakage faults on the circuits and appliances.

The thing that makes this kind of fault tricky is that any of the individual faults that are contributing to it might not be large enough to be considered as a problem under normal circumstances. From the homeowner point of view this type of fault is very confusing because unplugging or disconnecting any of the contributing faults might stop the tripping problem because the cumulative effect of the remaining faults is just under the point where the RCD would trip.

Here’s an example. In a house with a 30mA RCD there’s functional standing leakage of 15mA from all the 5 PC’s, 3 laptops, router, decoder, HiFi and other electronic items. There’s also a washing machine with 12mA leakage, a pool pump with 1mA leakage and a gate motor with 2mA leakage.

If everything is plugged in you have 30 milliamps of earth leakage current which will cause the RCD to trip. Unplug the pool pump and the RCD will just hang in there and not trip because the leakage is now 1mA less than the amount it will allow. You now take the pool pump for repair because you think it was the cause of you tripping problems, the repair guy bench tests it and only finds 1mA of leakage, he signs it off as being in good working order, shrugs his shoulders and gives you an invoice.

You hopefully see the problem here. Even removing a small part of the overall leakage current (pool pump) has fixed the tripping but the appliance with the biggest earth leakage fault (washing machine) is still connected. The same would happen if you unplugged the gate motor first, it also would appear to be the main cause of the tripping even though the earth leakage it’s causing is actually acceptable.

Where this is all going is that there’s very little you, as an electrically unskilled person and without test equipment, can do to effectively find or localise this type of fault. You’ll need an experienced electrician who has the right test equipment.

[AndyD]

What Should I Expect From My Electrician

First step would be to test the RCD itself. As I previously mentioned the chances of the RCD being faulty are very slim but it should be tested as a matter of course none the less.

To test an RCD properly you need a special RCD ramp tester (see second picture below) and it would be worth confirming in advance with the electrician that he owns one. They're generally very expensive (>R20K for a Fluke) and many electricians make do with a very basic tester shown in the first picture that is not capable of performing the full range of tests. If he doesn’t own a full ramp tester then rather find an electrician who does.

Not An RCD Tester



RCD Ramp Tester


If the RCD passes the barrage of tests a ramp tester throws at it then the fault is definitely on one or more of the circuits it's supplying. Second step is to insulation test (IR test or Megger test) each circuit and all the items that get plugged in to check for an insulation fault that would cause an earth leakage fault current to flow.

Megger Tester

The thing with insulation tests is that there's no 100% pass, every circuit has got an insulation value that's less than infinite so every circuit has an earth leakage fault current even though it might be very small. The problem with very small leakage currents is that the RCD sees the cumulative total so if you've got 5 or 6 circuits each with 5 or 6 mA earth leakage then it's already game over because the total is >30mA even though no individual circuit would fail as having an unacceptable leakage current.

This is often where the lesser sparky calls it quits. If he's tested the RCD itself and insulation tested the circuits and there's no fault large enough to peak his interest, this is all too often a good time to get the invoice made out and hope for the best and head for the hills.

In the good old days you could do the insulation tests and from the results you could calculate your expected leakage current under live conditions and the calculations would usually have been pretty accurate. Zoom forward to the 21st century and you now have electronics everywhere and every house or business has dozens of electronic power supplies (switch mode supplies) and even under normal operating conditions they have a small amount of leakage current, it's generally one or two milliamps for each power supply but the RCD unfortunately views this current as a fault and a megger tester can't give any indication of how large these currents are or whether they're large enough to be problematic.

So... the next step for me would be a live leakage test which involves another relatively expensive tester, an earth leakage clamp meter which any good electrician should also own.

Earth Leakage Clamp Mater

The megger tests/insulation tests are dead tests (power is off) and whilst they provide valuable information about the condition of the circuits and appliances they don't tell you how much leakage there is in real-life. An earth leakage clamp meter around the live and neutral on the load-side of the RCD as a live test (power on) will indicate of the difference between the current flowing in the Live and the current flowing in the Neutral. This difference in currents is leakage and it's exactly what the RCD monitors. If that difference is > than 30mA then it trips, even over 25mA it's likely to cause occasional nuisance tripping. The live leakage test needs to be done over a long duration with a tester that has a 'max hold' facility so you can see the maximum level of leakage current that took place over time.

The other advantage of doing the live leakage test during actual real-live conditions is that many modern appliances have internal components that are controlled by 2-pole switching. This means that when there's no power on the appliance, any insulation fault that may be present would not be visible. Live leakage testing will find the fault even if it takes some time.

Finally the incoming supply should be inspected and tested. The incoming supply is unlikely to be causing an earth leakage tripping problem but there are certain main supply faults that possibly could, depending on earthing arrangements etc. It’s also good practice to make supply tests any time a DB is worked on just in the interest of safety, it takes about two minutes to do the tests and it’s peace of mind the earthing is good and the installation is safe.

A good electrician should be inquisitive, he should be checking in the ceiling, he should be rummaging under and around things, he should be removing lids and covers and he should be all over the installation looking for possible problems inside and outside. There are certain faults that all the test equipment in the world won’t find, the only way you discover them is by getting down and dirty and hands on and by being methodical and persistent.

Alas, not all electricians are made of the right stuff. All too often we’ve attended a fault after several electricians have been there before, only to find a litany of bad practice and sometimes downright dangerous ‘repairs’ where the RCD has been bypassed or a faulty circuit has been removed from the RCD as a workaround rather than going to the trouble of actually repairing the root cause of the tripping. The only advice I can offer is to find an electrician who comes genuinely recommended by other people and don’t necessarily go with the guy who has the cheapest hourly rate.

The info above is to give you a better idea of what you should be expecting from your electrician. These tests are not within the realms of DIY, they're dangerous if you're not electrically competent and even with qualified electricians live testing should be done only after an appropriate risk assessment.

Good luck with your tripping faults and stay safe.

[AndyD]

If there's anything I missed, anything that's inaccurate or anything that doesn't read too well grammatically please let me know.

[Leecatt]

If there's anything I missed, anything that's inaccurate or anything that doesn't read too well grammatically please let me know.

Well I don't know what to say. That has to be the most comprehensive narration on earth leakages I have ever read, even I learned a few things and I've been in the game over 40 years.
If you have no objection Andy, I would like to plagerise your work and put it into my website.
Well done.