No idea about the harmonics (that was skipped over during my course), but IT equipment has a measurable line to earth leakage. It is something to do with the transformers in them, which leak a little. Add 10 computers in one room, and there is potential to trip an RCD.
There are special rules in the 17th Ed. Regs. for rooms containing IT equipment, especially concerning their earthing - not earthed properly, and you then touch the casing of a computer, you can get a shock, so all earth connections have to be in separate terminals,and must be in a ring, even if the line circuit is a radial.
This morning I connected the vacuum cleaner directly up to the RCD in the consumer unit. Nothing else connected. Immediately I turned on the vacuum cleaner, the RCD tripped. So, I have reproduced the fault having isolated the wiring.
Bearing in mind the above, and the fact that I have seemingly random trips occuring on all RCD protected circuits, at inexplicable times (i.e. no load), I am inclined to suspect the RCD as being over- sensitive, and am minded to buy a replacement.
I am pretty sure I am right, that a vacuum cleaner, which presents no path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it?
Its usually capacitors connected between Neutral and earth and Live and earth. Not a lot of current is intended to flow but some does. These are mainly used to stop or alleviate interference (radio frequency) problems often referred to as EMC compatibility. That's it in brief..
Thats the idea .. what goes into the "house" on one conductor must come out on the other, any discrepancy then thats "leakage" sometimes caused by capacitors in equipment, sometimes caused by insulation breakdown and sometimes caused by YOU when you might grab hold of a live conductor and the leakage makes its way to Earth by you .. not a good thing then the RCD trips out and disconnects you from Earth..
No theres no fooling at all as such, what comes in must be the same as what goes out more then 30 milliamps difference - then trip out time.
Sometimes an Earth Neutral short can be rather difficult to comprehend as normally with no current flowing then nothing will happen as Earth and Neutral are bonded together at the incoming point on most supply instances. However when some current starts to flow then as the current increases what is going in on the live isn't all coming out on the Neutral thru the RCD as the current thru that increases due to load then there will come a point when it gets past the trip value then out will go the trip!..
If no current is being consumed in the connected circuits then the current imbalance isn't enough to fire off the trip. Switching anything on that will cause sufficient current to flow will .. after the tripping current point is reached then out it goes.
Put that in a simplistic way with nothing switched on then nothing will be flowing thru the live to come out on the neutral so no imbalance.
However switch something on then the current flowing Into the RCD on the live it should see an equal current flowing OUT on the Neutral which it won't as its flowing away through the Earth system, not necessarily to earth as such, just the earth part of the incoming supply connected to the neutral now flowing around and past the RCD on the wrong conductor
When there is sufficient current flowing thru the live conductor and not out of the neutral side then once that reaches the tripping current then it will trip out...
No its not if its a Neutral Earth short, then that can cause that with no problems at all as can a lamp or anything else that causes currents to flow.
The only exception might be a faulty or leaky "suppression" capacitor that is permitting a current to flow from the Live wire in it to Earth that can trip it.
Way to find out is to tray another load such as an electric fire on the same circuit, tripping then Earth Neutral short very likely, no tripping then prolly faulty caps in the vacuum cleaner..
Was this wired directly into the RCD with all the other connections to the RCD removed (apart from the incoming L and N supply) or was it wired into the RCD with the outgoing neutral still connected. It does make a difference.
OK, so the RCD measures the current flowing in the live wire and measures the current flowing in the neutral wire and compares the two. Notionally, if the current in the neutral wire is less than the current in the live wire, then some of the current from the live wire *must* be flowing to earth, as the neutral is bonded to earth at the substation and therefore earth provides an alternative return path to neutral for the current from the live wire. If the neutral current is lower than the live current by a threshold value, then the RCD trips out the supply.
Some types of equipment can inject some nasty signals into the mains which cause interference with other equipment, so to comply with the EMC directive, suppression components have to be installed to stop the interference from getting out of the unit.
One of the techniques for limiting the interference is to connect small capacitors between live and earth and between neutral and earth. These are called Class Y capacitors[1].
Now, some small current flows in these capacitors. As the nominal potential difference between live and earth is 240V and the nominal potential difference between neutral and earth is 0V, then the current flowing from live to earth is 240V/Xc and the current flowing from neutral to earth is 0V/Xc (i.e. zero), where Xc is the impedance of the capacitor.
So, with this type of interference suppressor, current (say 0.5mA) legitimately flows from live to earth and this current is detected by the RCD. These currents all add up, so, for example, thirty pieces of IT equipment[2] may have a leakage of 15mA. If the nominal 30mA RCD actually trips at 25mA then effectively, RCD protection is sensitised to
9mA of unacceptable fault current instead of 24mA of unacceptable fault current.
Now, the impedance of the suppression capacitor (Xc) is a function of the frequency of the signal passing through it. As you will see from
formatting link
the impedance is inversely proportional to the the frequency, which is another way of saying that when the frequency rises, the impedance falls.
So if the device being protected generates harmonic interference (i.e interference at multiples of the mains frequency), then the harmonic interference is going to cause an even higher current to flow between live and earth than that just due to the 50Hz mains.
In summary, there are circumstances where a legitimate current flows from live to earth; these circumstances are commonplace in a modern house and they cumulatively serve to "sensitise" the tripping characteristic of the RCD.
[1]
formatting link
How many pieces if IT equipment in your home?
Computers? Laptop power supplies? TVs? DECT phones? DVD players? Satellite boxes? Printers? Scanners? Monitors? ADSL modem? Phone chargers? Microwave oven? etc.
To answer your question, I left all the neutrals connected to the neutral side of the RCD but only connected one live, which was to the vacuum cleaner. I would take a photo so you could see exactly what I did, but I have disconnected it now and don't want to cut the power at this time of night to recreate it.
So, I decided to simplify matters yet further by wiring the RCD into a socket:
(Health and safety fanatics look away now)
formatting link
this time the RCD didn't trip, and the vacuum cleaner worked fine. I must admit I am confused as to how leaving all those neutrals connected could have been a contributory factor, as only 1 live was connected. Is it therefore possible there is a leak between a Live on the non-RCD side and Neutral on the RCD side then? If you like, I will re-execute my original experiment tomorrow morning and post a picture.
You have the symptoms of a classic neutral earth fault on the RCD side of your house electrics.
The RCD needs the live and neutral that pass through it to have the same current, any imbalance more than 30mA and the RCD will trip.
If the neutral and earth are shorted after the RCD (in the T&E house cable not the vacuums flex) then some of the current that should flow down the neutral cable back to the RCD is flowing to earth and the RCD will trip.
Older equipment, with transformers, doesn't usually do this. The real problems began with the advent of switch mode power supplies (c.1980): a way of building power supplies out of complex silicon (which is cheap these days) and small high-frequency transformers, rather than the old heavy 50Hz transformers. As these are high-frequency devices, they make electrical interference that can be transmitted to, and interfere with, other equipment.
The solution to this (for both interfererence generator and susceptible equipment) is to fit input filters. These act as short circuits to high-frequency noise, shorting it out and dissipating it between live, neutral and earth. In practice they're usually five components: a pair of inductors or chokes in the live & neutral lines act as an open circuit to high frequencies whilst a triangular delta network of capacitors between the three wires, including the earth, acts as short circuits to high frequencies.
Capacitors are an open circuit to DC and nearly so to low frequency
50Hz AC, so they don't usually trigger RCDs, certainly not when it's an older RCD of 100mA. However, as John Rumm points out, it's a cumulative effect and 30mA isn't a huge leakage allowance to begin with.
Harmonics isn't the right term. The thing is that if you switch an inductive load (such as a transformer or motor) and you don't switch exactly on the zero-crossing point (which only happens with sophisticated transistor or thyristor switches) then you're attempting to switch a sizable voltage instantly. With an inductive load, that would require an infinite rate of change in the current, which an inductor will resist. So the outcome instead is a "ringing" current, where the current and voltage oscillate until damped out. The extra current drawn during these oscillations is also out of phase between live & neutral (as they're on opposite legs of the inductive load), so it's seen by the RCD as if it was a leakage current (it isn't, it's purely between live and neutral, but it's still picked up by the RCD's differential transformer). This is a relatively slow oscillation, compared to that of a switched-mode supply's oscillator, so RCDs are still fairly sensitive to it.
That's not what happens. Current cant be 'lost' in an inductor or a capacitor. What goes in comes out.
Its purely down to the capacitative coupling between L&N, and E.
There is a current path via any interference suppression capacitors between those and earth. Now at 50Hz sine, that's rated as a very low current BUT if you slam a big load on the mains, you get a sharp step in voltage (equivalent to a much higher frequency than 50Hz being applied) on the mains, and that translates to a big surge current through those capacitors.
In short, if you have maybe 20 electronic bits plugged into the mains, and you connect a powerful motor and switch it on, you are almost guaranteed to flip a 30mA RCD.
That, and neutral earth shorts are the most common causes of phantom tripping.
When I had a lorry sized generator instead of a proper mains supply, I could reliably flip the RCD any time switching on a hoover or kettle.
Well one thing to consider with this rather suicidal lashup, is you don't appear to have any earth connection. Since RCDs trip on current imbalance, you would need an alternative path for current to flow through other than L & N to get a trip. Since you have disconnected the earth, you have also lost the most likely route (unless you have the vacuum cleaner itself sat in a bucket of salt water resting on you lap off to the side?)
Remember that the neutral bar for the RCD side, and the one for the non RCD side should only connect to neutrals on circuits fed from the matching live side. Any crossovers will certainly give you a trip.
A typical input filter will include (as a minimum) a couple of capacitors that bridge L to E and N to E. (they will probably also include an inductor or two inline with the supply wires - but this is less important from a leakage point of view). The purpose of these capacitors is to snub high frequency AC that is generated inside the switched mode power supply and prevent it escaping out of the mains lead and into other parts of the electrical supply. As you probably know, the impedance of a capacitor in a AC circuit is proportional to the frequency, and it can be found with the formula:
1 Xc = -------------- 2 x pi x f x c
So by careful choice of capacitor value, you can pick one that has a low impedance for the sorts of high frequencies you know will be generated inside your PSU, while looking like a very high value at the nominal
50Hz of the mains itself.
This means that any HF noise generated in the PSU that makes it way back out the mains lead, will be coupled to the earth wire, but the input mains is largely unaffected.
However there are some undesirable side effects here with RCDs. Firstly it does mean that some of the energy entering via the mains, will be leaving as "leaked" HF noise via the earth, and also any HF noise present on the mains input itself will also go the same way. (this is one reason why switch on surges, or mains spikes and sags can cause RCD trips [1]). Secondly, although the filter will have component values selected such that they are high impedance at 50Hz, there will still be a very small current flow that makes it directly from the mains through the filter.
When some of it finds an alternate path - either via the earth wire, or via some other path to earth like you (which is when the RCD jumps into action and keeps you un toasted!)
There generally are real leaks.
(There can also be some situations where you can fool the RCD - but those a distinct)
[1] The distortions all share a characteristic in that they can introduce sharp steps and notches in the mains waveform. Some analysis of the waveform will show that these discontinuities in it also contain large amounts of HF noise. This is yet more energy that will find its way via the filter caps out of L or N and into the earth connection.
(For anyone not familiar waveform analysis, there is a handy bit of maths called Fourier analysis, which demonstrates that a wave shape of arbitrary complexity can be created by adding a series of simple sine waves. So a bit of nice 50Hz mains with a step in it, like a squared off peak or brief interruption, can be shown to have an associated burst of other much higher frequencies contained in it.
formatting link
that example, if you add together the blue sine wave, plus the first three smaller "harmonic" waves, you will actually get the resulting purple wave - which is beginning to look more square (adding in a few more harmonics would make it look ever squarer)
Cable capacitance (which is pretty damned small), or (as I described) the filter capacitors?
We're talking about inductive loads here - so you _don't_ get this sharp step (which would indeed require those harmonics), you get a slower rise instead, with overshoot and ringing.
OK, understood. I've no experience of Hager products, so thanks for the info. Do they guarantee a minimum tripping current though, other than that implied by compliance with EN 60898?
Indeed, but I still think that the prudent fixed installation designer should assume a possible trip current of 15 mA and divide the installation such that no RCD should see more than (say) 10 mA of Idn leakage under normal load conditions (insofar as 'normal' can be predicted). Bear in mind BS 7671's requirements for maintainability - your Hager '27 mA' RCD could get replaced by a different make that trips at 18 mA, say.
Your own interest is more specialised and under 'skilled person' supervision, and I can see your need to avoid spurious tripping at all cost.
Interesting. I take it you've seen the ESC's reliability study:
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.