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Topic Title: Can an SPD trip an RCD?
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Created On: 10 January 2013 08:22 PM
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 10 January 2013 08:22 PM
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clansman

Posts: 180
Joined: 16 March 2008

Customer is complaining that thunderstorms are tripping one of his RCDs.

2 thunderstorms in recent months have caused one of the ring mains to be tripped out and on one occassion defrosting the freezer as they were away on holiday. Its not a water ingress problem as far as I can see.

There is an SPD protecting the telephone (plugged into the relevant ring main). My thinking is that SPD diverts voltage surge to earth and hence would cause RCD to trip.

Can anyone confirm that?
 10 January 2013 11:41 PM
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M.Joshi

Posts: 212
Joined: 10 January 2003

The RCD should only trip if there is an imbalance between the phase and neutral on the circuits/rings that are connected. The only way that it would trip in a thunderstorm were if a surge voltage due to lightening were to be induced on either the phase or neutral thus causing an imbalance.

The surge protection device is doing its job as intended by grounding surplus energy to earth. If they have indeed been hit a few times through lightning strikes to the overhead telephone cable, the surge protective component may be worn and 'leaky'. Is it a Belkin one with a neon which indicates whether the surge protection element is still intact?

Is the telephone on the same RCD as the freezer?

Could it be a sensitive RCD? What make is it?

Have you tried switching it out for another?

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Edited: 11 January 2013 at 01:20 AM by M.Joshi
 11 January 2013 07:53 AM
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Pacific

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If they were away on holiday, how do they know the RCD tripped during a thunderstorm
 11 January 2013 11:23 AM
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AJJewsbury

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An SPD connected to L/N downstream of an RCD can certainly cause a trip - after all it's diverting current to earth and that current passed through the RCD coil, so there should be an imbalance. Properly, the 1st line SPDs are upstream of the first RCD (e.g. at the origin of the installation), so large spikes coming in from outside the installation are diverted to earth before the RCDs see them. Smaller local SPDs (e.g. on individual appliances) should be dealing with smaller spikes so there's less likelihood of tripping RCDs.

Using small local SPDs without "big" SPDs at the origin could well increase the risks of tripping RCDs.

There is an SPD protecting the telephone (plugged into the relevant ring main). My thinking is that SPD diverts voltage surge to earth and hence would cause RCD to trip.

Of itself, it shouldn't, at least in theory. If the spike current is coming from the telephone line, the RCDs don't watch that, and returning through the ring c.p.c. which again the RCD doesn't monitor.

In practice though, things can get a bit more complicated. The entire electrical installation can act as one big capacitor - L&N as one plate and earth (or other services) as the other - so can pass a spike of current even though there's no obvious conductive path.

So you might have the situation where a spike coming in along the phone line, raises the voltage on the installation's earthing system (as the phone line SPD conducts), which via capacitive coupling raises a pulse on the installation's N conductor and so a proportion of the spike passes to true earth via the supplier's earth rods - which RCDs see on their N coils.

- Andy.
 11 January 2013 02:48 PM
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clansman

Posts: 180
Joined: 16 March 2008

In answer to some questions:

Telephone is on same RCD as freezer - ring main
Unlikely sensitive RCD - Crabtree
Not tried switching to another - not sure how long till the next thunderstorm!
Customer knew thunderstorm because neighbours told them and could tell from times left on some appliances.

Would putting SPD at origin cure problem?
 11 January 2013 03:53 PM
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Zs

Posts: 3008
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Clansman, I have a very basic suggestion for you;

I am assuming that your RCD is an RCBO or half of a split load board?

My parents live in a TT village which has power cuts and tripping problems during storms. For info, they have voltage on the high side of about 248 and a Ze of about 22 ohms. Often the RCDs in the village will trip as power is restored. They have three SPDs on two computers and a telephone, on two circuits.

A while ago I moved their DB while they were away, simply to get it down to a standing height for them and to eliminate their climbing on a small cupboard to reset what was a single front end RCD. Perchance, I also changed them to a split load 17th at the time and from wylex to hager.

For whatever reason they no longer have tripping problems. They still have the SPDs and they are all on the same side of the board. Word went out that I had done something magical for them. I am finding the same as I work my way around the village changing DBs. Hager doesn't seem to trip in a storm or on restoration of power. MK does and so does one other brand which I cannot remember accurately but it's CED or CDE or the like. Obviously, being a TT village the Ze's vary enormously.

Might be worth a try if you are looking for a possible quick solution whilst getting answers to your question? I have not done an all RCBO installation in the village yet so I'm not sure if the same applies.

Zs
 11 January 2013 05:35 PM
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jcm256

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From what I can gather when a lighting strike occurs the surge voltages increase, MOVs respond in just a few nanoseconds switching from the normal "Open Circuit" to a "Short Circuit" state, so fast the SPD switches on and off before one mains AC cycle therefore the mains fuses have not time to respond to the short. However I would have thought being electronic and fast, the RCD would trip. However did you fit an S Type, This is what Furse says:

SPD in conjunction with RCDs
SPDs installed before or upstream of RCD - no tripping problem from overvoltages
SPDs installed load side of RCD - nuisance tripping possible
RCDs to have immunity to surge currents of 3kA (8/20 surge current waveform)
S Type RCDs meet this requirement


Only for infotmation: Although American, Lightning the same all over the world

Surge Current:
When an SPD senses a transient, the surge components will begin
to go into conduction. This is required to divert the high-current
impulses away from the load and eventually to ground. Surge
impulses are fractions of a cycle, lasting only microseconds
in duration. The amount current the SPD can safely divert is
called the device's surge current capacity. This value is generally
calculated by summing up all the surge components within
the device. The surge current rating is generally measured in
thousands of amperes (kA) and may be published per each
mode, per each phase or as a total amount for the entire device.
Short-Circuit Current:
As with any electrical load, SPDs needs to have some type of
current-limiting component such as a fuse protecting it against
internal faults. These occurrences generally happen when a
surge component fails short-circuit and large currents flow for
an extended period of time. If left uninterrupted, the failure to
the device can be catastrophic. The amount of current available
to flow through this failed component is driven by the power
system.
SPD failures are generally caused by the following:
. SPD exceeds the maximum surge current capacity
. Misapplication of a product for its voltage rating
. Sustained over-voltage events
Fault Current:
Sometimes fault conditions happen outside the device in the
power distribution system that the SPD needs to protect itself
against.
These faults could be caused by two or more phases coming
in contact with each other causing a "phase fault" or "phase
short". The outcome is large levels of current flowing through
the power conductors, circuit breakers, SPDs and any other
devices connected in the fault path. Unlike fast-acting surge
currents, faults can last a quarter of an AC cycle or even longer.


http://www.emersonnetworkpower...1_singlepgs.pdf


Who is responsible for protecting the smart meters, would you need to place the class 1 SPD upstream or downstream from the meter.
Regards
jcm

Edited: 11 January 2013 at 06:24 PM by jcm256
 11 January 2013 05:44 PM
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AdrianWint

Posts: 265
Joined: 25 May 2006

I to believe that it is possible for a SPD to trip an RCD.

I had an experience a few years ago of a direct hit close to our home. This occurred in the early hours of a summers morning and it hit the metal ridge of a small shopping complex about 1/4 mile away. I'd already been awoken by the rain & I was lying in bed listening and watching the light show. There was very very bright flash with an instant tremendous bang. At the same instant we lost power.

Going to investigate I found that the main RCD had tripped as well as the 32A MCB which supplied all our computing/networking kit. This circuit is covered by several plugin type SPD's and well as a couple of UPS.

What I think happened is that the EMC pulse from the close hit induced a voltage onto the mains/our wiring ... this was picked up by the MOVs in the SPDs (two of which where destroyed) which started to conduct, this was seen by the MCB as a high over current which caused it to operate in its magnetic region. This high current pulse would, probably have saturated the balancing coils with the RCD causing it to false trip.

After resetting & replacing the blown SPDs everything worked normally.

So, yes, think that SPDs can trip an RCD, not because of leakage to earth but because they are capable of causing large magnitude current pulses to flow which can saturate an RCDs sensing coils.

Adrian
 11 January 2013 06:37 PM
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Zs

Posts: 3008
Joined: 20 July 2006

Originally posted by: jcm256



Zs have you no heart,

Ha, I reckon I do have one, albeit too oft broken and held together with sticky tape and string.

the government thinks the smart meters are the bees-knees,


Did you not consider protecting it being packed full of electronic things, (if one fitted) in your design.

No I didn't. I was just moving the DB to stop them from falling down or using a broom-handle to re set. I found they had made something out of a bent cane so as to be able to reach into the back of a cupboard. Actually, the power cuts during storms to which I refer are not very often lighting storms and I have lost a computer to lightning in the past, whilst living in a barn conversion about two miles from that village.

A lightning storm around here sends us all rushing to unplug sensitive devices. But I will give it some thought having read your american info, which is very interesting.

Regards

jcm


Heartlessly yours,
San Francisco Zs

OMG and way off topic, flashback. I did actually, July 1992 I remember exchanging the heart for a hired Pontiac(?) at the airport in SF and watching him drive away in some old car with an inexplicable black PVC cover across the headlights and grille. His name was Wayne-William the third, or was it junior? I think he thought I was going to fly home distraught. But there was this red Pontiac Grand Am outside Avis you see. . . So I drove myself to Cannery row, Malibu, Newport Beach,Harmony, and some place where they grow loads of onions and have an onion festival. But that was a silly place to leave a heart so I got it back eventually. Got done for speeding on that gorgeous highway between SF and LA. And now I'm a spark. Where did all that risk taking go? Cor jcm, you've made me smile.
 11 January 2013 07:02 PM
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OMS

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LoL - pass me a cigarette, I think there's one in my raincoat

OMS

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Failure is always an option
 11 January 2013 08:11 PM
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Legh

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Ha, I reckon I do have one, albeit too oft broken and held together with sticky tape and string.


I hold mine together with electrical insulation tape and grn/yell sleeving
but then I'm a bloke and we use the proper stuff

Legh

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http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 11 January 2013 09:17 PM
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Zs

Posts: 3008
Joined: 20 July 2006

Originally posted by: OMS

LoL - pass me a cigarette, I think there's one in my raincoat

We smoked the last one an hour ago OMS



Errr, have we sorted out Clansman's question?
 11 January 2013 11:03 PM
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clansman

Posts: 180
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Errr, have we sorted out Clansman's question?


Thanks, I think I'm going to suggest they unplug the telephone & PC when they go on holiday!

With cost of putting SPD at origin I'm not sure it's worth it and no guarentee it'll fix the problem!
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