Nuisance Tripping

Excess current protection devices, otherwise known as Circuit Breakers, are common place all the way from the generation plants to the home. It's when they break the flow of power, for no apparent reason, that they start getting a seriously bad name (and the user vowing to install the old faithful fuse!). Further to this, one circuit tripping can lead to others also detecting a fault and deciding it's 'time to take a rest'.

This is owing to the fact that when a protection circuit trips, the load is suddenly removed from the source. Depending on the size of the load, the impedance of the source, and the fault condition causing the trip, the source voltage can suddenly shift many volts inducing higher-than-normal currents causing further trips down the line - a chain reaction. Entire towns can lose power through this type of event with tripping circuits set slightly too sensitive.

It must be remembered that the breaker is usually mechanically connected across all three phases which will mean that during a trip a serious imbalance takes place within the mains cycle of the trip (especially with back-EMF from transformers). This fact exacerbates the chain reaction that can occur.

So having breaker protection is a two-edged sword in that the tripping results in varying loads on the network and therefore the quality of supply of that specific network. The other edge being nuisance tripping through bad power quality, the tripping in turn bringing us back to point one. A vicious circle!

But, first, I need to concentrate on a bit of bad teaching that is very difficult to erradicate (especially as it is kept alive by less-than-knowledgeable contributors to help forums!).


Nuisance trips are clearly defined as unwarranted circuit breaker trips with either no electrically based reason for the trips, or, the breaker deems there to be a fault when one does not exist.

Nuisance Tripping is not the tripping of a breaker when doing its designed function. It is very tempting to call an 'intermittent' electrical fault (such as an intermittent short on a power cable) a 'nuisance trip' - in such instances the breaker is performing the very function for which it was installed.

I have a devil's own job in getting even top-notch engineers to understand this difference between a breaker rightfully tripping because of an intermittent short or some wire that has been incorrectly placed during repairs, as opposed to irritatingly tripping because the breaker thinks there's a fault when none exists. It is a subtle difference, but an all-important one!

A classic example of this; The most common cause of (incorrectly named) 'nuisance trips' with RCDs are from misplaced or combined Neutrals; The trips usually beginning shortly after maintenance or modifications to the wiring. Either the Neutral intended for protection by the RCD is wired to the 'pre-RCD' Neutral bar, or current is accidentally shared between the 'pre-RCD' Neutral bar and 'post-RCD' Neutral bar (usually through some or other common bond that should not exist).

The next most common cause of RCD trips usually occurs when the RCD is fitted for the first time - they show up Neutral-Earth faults not previously known to exist. Suddenly, the job of fitting the RCD has turned from a mere simple rewiring of the distribution board into a huge fault finding exercise. Not a sparky's most relished thought!

In these instances the RCD tends to behave itself until any load is placed on the now incorrectly wired circuit. You may need to consult the explanation later on the workings of the RCD, but suffice for now to say that if the current taken by the load does not flow equally in the Neutral and Live conductors through the RCD it will see this as residual current and trip.

But this is not a nuisance trip! The RCD is correctly stating there is a problem. Insult is often added to injury here as the person who 'tests' the newly formed (faulty) wiring will do so with a meter or some neon-lamp style socket tester (which says "all's ok, Jack") and then wonders why when switching on some load it causes the RCD to trip!

These 'testers' draw extremely small currents and have no hope of tripping even a 30mA domestic RCD (the load, however, easily exceeding this current). How people like laughing when I test new circuits with a 100W lamp. But hey, the last laugh is mine!

In short; The focus here is not on faulty wiring (be it incorrectly wired Neutrals causing an RCD to object, or some intermittent short causing a current breaker to kick up a fuss). The focus here is on breakers tripping yet the standard tests revealing nothing. Thankfully, there is one common factor and that is the result is the same; The power is lost. So testing for both intermittent and nuisance trips is done in the same manner (see here).

Tracing why breakers trip for no apparent reason requires that we understand the basics behind the very devices that are prone to such tripping. There are effectively two types being the normal Current Activated Circuit Breaker and the Residual Current Device (RCD), otherwise known as an Earth Leakage Breaker.

Current Activated Circuit Breaker

Whether these are single or multi-pole, the method by which they operate is effectively the same. In most multi-pole breakers the breakers are mechanically interlinked such that any one pole trips, the whole breaker is forced to operate and disconnect load from source.

In all types, there are two contacts held together by a spring mechanism ready to pull the contacts apart at a moments notice. It is the "notice" that changes in flavour between magnetic or thermal.

With magnetic, the current is fed through a small solenoid that, when the current exceeds a prescribed value, the 'pull' on the solenoid releases the spring spoken of earlier and the contacts fly apart. With thermal, the current is usually fed through a bi-metal strip that bends in relation to the current. Again, should the current exceed the prescribed value then the spring is again released and the contacts made to part company.

There is one advantage to industrial version thermal (and thermo-magnetic) breakers and that is the "release current" can be adjusted to some degree, much like the temperature control on a clothes iron. This allows for the breaker to be set to break as soon as a current is exceeded (not forgetting the thermal delay). The advantage is only one breaker type needs to be specified for a wide range of applications.

There is an interesting breaker in this range known as a thermo-magnetic operated circuit breaker. These are extremely handy for loads that have large inrush vs. operating currents. The obvious choice for motor starts. The magnetic can usually not be adjusted, but the thermal can. They are usually specified at about 10:1 inrush to operating. On a largish motor an example could be 200/2000A. The operating current is therefore a maximum of 200A, but may have an inrush of 2000A.

Should the inrush be exceeded (e.g. a dead short on the cable) then the magnetic portion will force a trip. Should the motor be taking long term strain, or remains stalled, then the thermal portion's bi-metal strip will start to bend and force a trip. One other advantage is if the breaker is set correctly, then should the motor be taking strain such as a siezed bearing causing the current to rise, then the thermal portion can again protect the motor.

Nuisance tripping with these breakers occurs through two primary factors. The first is the human. Here it is simply that the wrong breaker was specified for the job. A prime example is using a magnetic breaker in a motor controller. Nuisance tripping will be recognised by a trip occurring on motor starts.

But magnetically operated breakers are also not suited to most electronic applications where there is a large current crest factor. I will never forget an electronics 'expert' being thoroughly confused when fitting a relatively tiny UPS to a telephone tape recording rack (the type used in emergency centres) and the 10A (that's 2.3kW!) breaker tripped whenever the UPS was in circuit, but stayed put when the UPS was on bypass.

Replacing the breaker yielded the same result, and raised the level of confusion! The give away was the breaker started to buzz furiously a few minutes before tripping. Showing the poor chap what the waveform looked like and that, although the RMS was well below 10A, the peak far exceeded the 14.14A that the magnetic portion of the breaker was designed for. Yes, a thermal breaker solved the problem.

The second reason for this type of breaker tripping is simply what I have termed "A Lazy Breaker". They are mechanical devices and are prone to fatigue as is any other mechanical device. Even if the breaker has been in service for years and has never tripped, it could suddenly decide to change personality. The springs get weak. A contact leaf cracks. The bi-metal strip decides to not return to rest with no current. The list is endless.

The result is the same. The breaker trips for "no apparent reason". Even thermo-magnetic breakers can start tripping on motor starts with the magnetic portion becoming lazy, yet the thermal portion still working perfectly. One such incident happened and was overlooked because the investigator set the power quality recorder up incorrectly.

But it is not just the magnetic portion that can fail. The thermal portion can become faulty or lazy too. The primary cause is the bi-metal strip, over time (rust, etc.), starts to become more sensitive to current than when first manufactured and starts to operate the breaker without due cause. Any slight contact failure too can aggravate this as any loose contact will generate heat and cause the bi-metal strip to think it has a job to do.

Residual Current Device / Earth Leakage Circuit Breaker

... and what our dear friends across the 'Pond' call a "Ground Fault Interrupter - GFI".

This device is usually made from the Live and Neutral fed together through a common core. On this same core is wound a secondary that feeds a small electromagnet that, when powered, will cause the breaker to "trip". In a perfect world what goes out on the Live should be returned on the Neutral, therefore the magnetism inside the core is cancelled and nothing appears on the secondary.

Sadly, we don't live in a perfect world and my definition of leakage actually exists in reality; this being that currents will use undefined paths on their way out of and return to the source. With the current being unequal, the uncancelled magnetism permeates the RCD core, this feeding the secondary, this feeding the electromagnet, this then causing the breaker to trip. Ok, all sounds relatively simple and so endeth a very basic lesson on the "mechanics" of RCDs.

Leakage as a result of capacitance is the major player in all of this. Although the wiring's capacitance plays a significant part, modern appliances "take the cake" with all the filters found on them! Even washing machines can boast a minimum of 4.7nF capacitors from both Live and Neutral through to Earth. It has been found for this to be as high as 10nF on some devices.

Because Live wiggles about, and Neutral doesn't, means the capacitance found between Live and Earth will conduct current while the capacitance between Neutral and Earth does not (because there is, relatively speaking, much less voltage between Neutral and Earth as opposed to Live and Earth). This creates an unbalanced and therefore uncancelled current in the RCD's core.

So what does not help is RCDs protecting circuits feeding lots of hi-tech equipment (and nowadays this means the modern home too). Revisiting the page on Leakage (especially hi-tech devices with EMC filters) will show these contribute to the unbalanced currents seen by the RCD. This effectively 'biases' the RCD i.e. will reduce the genuine leakage current required for the RCD to trip.

Depending on the level of leakage, RCDs can be found to be "living on the edge" where it takes just a small amount of extra leakage, such as the switching on of an appliance thus upsetting the balance for just a tiny moment, and 'thwack!'.. the RCD trips. The RCDs would usually accept these tiny upsets, but being biased by all the capacitive leakage minimises the chances of the little 'blip' remaining below the RCD threshold.

With respects to "living on the edge", there is one thing not commonly known is that RCDs will trip when subjected to continuous current of about half their rated trip current i.e. 15mA on a 30mA RCD. This is to ensure compliance with required "speed of trip" under human touch faults.

Also, if a dual-function breaker (RCD and over-current breaker in one), there is just a feint chance the breaker has become weaker (over-sensitive) over time - especially if subjected to a ream of violent trips in its life. This is, however, said with reservation as I have not yet experienced such an RCD (and have only heard of the odd one) in my life, but I'm not saying the situation does not exist.

Let us now examine noise! If the noise on a mains feed is differential mode (i.e. superimposed on the waveform e.g. transient) then the current would go out on the Live and return on the Neutral. As these are fed through the same core with the currents flowing in opposite directions, they would cancel - in other words nothing would happen.

At first glance common mode noise, i.e. when the same noise appears on both Live and Neutral in phase and amplitude (a full explanation of this appears in common mode noise) is not an issue either as it makes no difference to the RCD that both the Live and Neutral (as a pair) are at some potential above Earth.

When one adds in the capacitance, however, the situation changes. With common mode noise the Live and Neutral 'move', as a pair, relative to Earth causing currents to flow from Live as well as Neutral through to Earth. The problem is these currents flow in the same direction and therefore combine in the RCD core rather than cancel. The result of this, well, I think you've guessed.

Wiring capacitance is relatively small meaning the common mode noise would have to be rather large to have this effect on the RCD. Again, the filters on modern appliances sure don't help and drastically reduce the level of common mode noise required to cause a trip (not just through the L-E and N-E capacitors, but also because they strap a rather large capacitor across L-N forcing the two to move toegether!).

Till now it has been said that Live and Neutral move, but the same effect is seen should the Earth move with respect to Live and Neutral. Nuisance tripping can therefore occur with sudden shifts in the ground as, through leakage, common mode currents on the Live and Neutral again add in the RCD core and, if large enough, will be detected as an imbalance and trip the RCD.

Such tripping is highly possible where Earths are weak and/or tied to surrounding earth points like water pipes etc. i.e. points where noise can be introduced into the system. As is, T-T systems have no connection between Neutral and Earth making them exceptionally vulnerable to nuisance tripping through common-mode noise (unwanted voltage between Live & Neutral as a pair and Earth).

With TNC-S, the Protective Earth is derived from the Neutral as it enters the property. At this point the Protective Earth is also attached to the nearest water and/or gas line. The Live and Neutral are fed through the RCD and the Protective Earth is not. Should the Live and Neutral not follow the same path to an appliance as does the Protective Earth (hot water immersion heater is a prime example, even a washing machine would do it!), then it is possible for the RCD to be part of a large magnetic circuit. Induce fast changing magnetic fields into the circuit and the currents will manifest in the RCD.

Other possibilities are defective ring circuits. If the Live and Neutral are not each a complete ring i.e. magnetic fields are not cancelled, it is possible for noise to be generated on to the circuit. The RCD will see this as an imbalance and, again, trip.

Low current RCDs, such as found in domestic properties, can, owing to their higher sensitivity, trip because of a fault ahead of the point at which they are installed i.e. external to the premises. With the widespread use of EMC filters, an intermittent short or significant load shift on the input to an RCD will trip the breaker owing to the unbalanced currents as the filter capacitors are charged and discharged with the input voltage variation.

In the following example, the disturbance was caused by a set of streetlights being turned on. The waveform clearly shows the immediate load the discharged power factor correction capacitors placed on the source. This higher frequency disturbance was conducted from the Live, through the filter capacitors on hi-tech gear within the property, and through to Earth. According to the RCD this is, correctly so, seen as an earth leakage current and activated the trip.

There may be an argument that the Neutral should present an equal but opposite waveform, based on the fact the Neutral and Live conductors have similar properties. This is rarely the case. In older installations the Neutral has usually less core area than the affected Live, and in modern installations up to twice the core area of the Live - this all creating a platform for unbalanced levels of disturbance on Live and Neutral therefore unbalanced currents in the RCD leading to the trip.

Further Live-Neutral waveform imbalance occurs as a result of the Neutral waveform being 'dampened' by loads between the opposing phases and the Neutral. This too opens the door to a large load change on an opposing phase causing only the Neutral to shift and therefore, again, causing unbalanced currents and RCD trips.

Although the above example trip was as a result of the momentary short placed across Live and Neutral as the discharged PF correction caps were switched into circuit, the exact same disturbance can occur with a loose connection ahead of the RCD on either the Live or Neutral. The loose connection on the affected current carrying conductor will cause a large amount of noise to occur on only this one conductor.

Although it is not incorrect to think this noise will return via the other current carrying conductor(s), some will return via the Earth because the capacitance between the current carrying conductors and Earth (especially EMC capacitors) becomes a low impedance at the higher frequencies of the noise. The current is, therefore, not cancelled within the RCD core and, again, results in an undesired trip.

Nuisance tripping (as shown above) is not just limited to serious faults where there are large changes in voltage. If there is sufficient unbalanced noise on the Live and Neutral i.e. the Live and Neutral do not have noise superimposed in an equal but opposing phase, then this will be 'seen' by the RCD as an imbalance especially if high amounts of leakage is 'biasing' the RCD.

What sure does not give the RCD a fighting chance is when such imbalance in the Neutral is created with the use of TNC-S style systems and especially where the Neutral is bonded to Earth at the entrance to the premises. Any noise on the Neutral is transferred to the Earth thus giving the Earth a similar potential to Neutral (the local Earth usually has a higher impedance than Neutral and easily takes on this potential).

TNC-S causing nuisance tripping

The differential mode noise (simply the turning on of a significant load in an adjacent property), which would usually cause equal but opposing disturbances in the Live and Neutral and therefore cancel, is now found as equal and opposing disturbance across the Live and Earth.

This, therefore, presents a case where the Live and Neutral currents flowing through the RCD to Earth (via the filter capacitors) are not equal and opposing, and will be again be interpreted by the RCD as an earth leakage event. In this case the simplest cure is to employ an RCD that ensures the leakage is sustained before activating a trip (please note we do not state removing the RCD!).

This situation can be exacerbated with T-T, Z-T and I-T systems (see here) as the weaker earth systems can easily introduce noise and voltages into the RCD core in the form of unbalanced or common mode currents, especially in the presence of ground gradients (voltages that exist as a result of current flowing through the ground).

Such systems are extremely prone to RCD trips during lightning storms as a result of ground gradients even with cloud-to-could strikes (there is this weird thinking that only cloud-to-ground strikes cause ground voltages!).

But, it does not need a lightning storm; It could well be a neighbour with a problem causing an RCD to trip. If the neighbour has some Live-Earth fault causes voltage gradients in the surrounding earth mass, this is picked up by the T-T system's earth rod and transferred into the RCD as common-mode currents.

A word about 'Electronic' RCDs; These devices, as opposed to the electro-magnetic-mechanical types, are extremely prone to nuisance tripping because they are designed to not only trip during imbalance but also trip if a fault is detected with the incoming supply. Unfortunately, being electronic, these devices have very fast reaction times and will trip even on extremely sharp events such as switching transients. Many a case of nuisance tripping has been resolved by replacing the RCD with the more basic type.

Related Pages:
Detecting Intermittent/Nuisance Tripping

Impulse Arrestor Failure  >>

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© 15.07.04