We came home Saturday after a day out to find the main power breaker indoors had tripped and also another in a small CU in an outside building. We're at the end of a spur off a rural feed.
What sort of disturbance might cause two such breakers in series to go off and is there anything I can get done (probably by our friendly Sparks) to provide better protection? Do I need a faster breaker indoors, if such a think exists?
Depends on your set up.
TT supply?
Firstly, what type of breaker are you talking about? A MCB, RCD, or RCBO (post a photo if unsure)
A MCB will have a single "switch" and no test button. It will be marked typically with something like B6 or B32, where the number is the nominal trip current in Amps.
RCDs will have test buttons, and will normally mention a trip current that is much lower - say 30mA. They won't normally show a nominal current (but might include the maximum current - often 63A)
RCBOs combine the functions of both, and so have nominal ratings, and a trip current rating plus a test button.
As Adam mentioned it would help to know the supply earthing arrangements...
It sounds like you have one CU fed from another. There are various options - which are appropriate will depend on exactly what tripped and why.
A momentary interruption or disturbance on the incoming feed will often cause RCD's to trip. The series bit is irrelevant as there is no way of predicting with two similar trip time specification RCD's in series which will trip first or if both will.
Not really.
No. Even if you could find such a device it would only make things worse as it would be more likely to trip with transients and in tripping would often cause the downstream device to trip as well.
Putting RCD's in series is generally considered to be not a good idea as it isn't easy to achieve discrimination between them while still maintaining protection.
(Wiring Reg 531.2.9 discrimination between RCD's in series.)
If they must be used the configuration should be a ?S? (Selective) type (slower to trip) on the incoming mains followed by a general type, so on your incoming mains you put the Selective RCB, you use the output of that to feed BOTH (and only both) consumer units each of which would have a general type RCB fitted. This still wouldn't solve a trip caused by transients on the incoming wire problem.
Basically, on the end of a long rural line some nuisance tripping is always going to occur.
Ummm. Looks like I better study all that and post some pix. What we actually have is quite a mish-mash.
I can see a rejigging of the mess in the understairs cupboard looming.
Trips that happen when we're here are less of a problem, and I've got a UPS for the computers, so they don't see a small outage.
another thing is what was on at the time. Some time ago it was common to find a VDR across the mains to stop spikes in rural areas, say Wales, inside plugs or put in the gear itself. Because these devices can pass lots of amps fast they just blow the most sensitive trips.
I used to use them in the early days of computers. Crowbar protection is all over the place to protect stuff, so really you need to know what actually happened. I've ften thought some kind of interface to monitor the mains and report out of spec happenings could be a nice project. Brian
Someone I know lives in a rather rural location and nuisance tripping was a constant problem as he was often away for days at a time. He eventually resorted to making a self resetting RCD by fastening a cord onto the RCD lever and connecting that to a large solenoid. When mains failed the solenoid (connected to a battery) energised for a second pulling the RCD back on. If there was a real fault it didn't matter as the RCD can't be reset while tripped. If nothing happened it kept retrying at hourly intervals until mains came back or the battery died.. Not sure it met and wiring regulation but it saved him the contents of the freezer on occasion.
With careful design that does not have to be the case.
With careful design of the line I agree, but when it has growed like Topsy over decades with houses added and farms changed role or installed power hungry devices it is often unavoidable. My friend with the RCD reset was at the end of an overhead line which spanned a river valley with trees on both banks, fed a farm which switched something on at 17:00 each day which caused lights to dim for a few seconds and had had a number of barn conversions added to it. No matter what he tried transient events on the incoming line caused nuisance tripping. The line is supposed to be upgraded at some point in the next decade or so.
Even then, you need to consume a fair amount of the leakage budget of a modern RCD before transients alone cause tripping IME.
When I moved in here (TT setup, overhead line etc), nuisance tripping was quite common, but then there were lots of circuits, all fed from the same 30mA trip RCD. That also included all the garden circuits etc. i.e. inadequate design of the installation that was asking for problems.
Since I re-engineered the head end with two separate CUs (one for household circuits, one for outbuilding / garden), both split with cascaded RCDs (time delayed 100mA ones acting as incomer), nuisance trips no longer occur.
I use the 100mA trip side of the CU for protecting some circuits directly; such as lighting, and typical high leakage circuits (cooker, immersion etc), and also for feeding a dedicated RCBO for the kitchen / utility (another high risk circuit). Then the secondary 30mA trip RCD protects all the other socket circuits
All external circuits, and out building sub mains are taken from the second CU configured in a similar way.
If incoming transients are causing frequent tripping, something's wrong, and the install would not qualify as carefully designed.
NT
Good job he did not have a MK RCD that needs pressing down before truning back on!
Here's a couple of photos of the CU in the shed/garage:
It's the light grey switch that's got into the habit of popping. Whether that's due to the load caused by the garage doors, I know not.
Meanwhile, under the stairs, we have:
which shows (see annotation numbers in red/orange):
Main fuse (1), which has a thin red/black pair to teleswitch (2) and thick red/black pair to meter (3). From the meter a red/black pair go to the main breaker switch (4), from which cables go to a distribution block (5). From this block pairs of cables go to various breaker panels including one for the garage (6).
(6) has never gone off (or at least not recently), it was the main breaker (4) and the shed/garage breaker that caused my OP. Since then the shed/garage breaker has gone off a number of times.
Apologies for any terminology errors.
Trust me - I have seen much worse:-)
That shows a 6A MCB on the left, a 20A MCB in the middle, and a RCD with a 30mA trip (or 0.03 A in a slightly Germanic way) on the right.
In spite of the labelling, it suggests that both circuits are protected by that RCD.
No, the light grey switch has no over current trip mechanism at all - the 63A rating is just the maximum current the switch will handle - draw significantly more and it will just melt rather than trip!
6 is basically configured as a "switch fuse" - i.e. it provides fault current protection to the feed to the garage.Tis ok, the pictures make it clear...
The "switchfuse" CU appears to have a 32A MCB in it - I can't quite read if its a type B or C - but its not really important in this case. Its a step larger than the 20A in the garage, and so will probably discriminate in the case of an over current in the garage.
Your trip in the garage is an RCD trip caused by earth leakage. That could be a fixed wiring fault, or it could be an appliance fed from one of those two circuits.
What devices are on and always powered in the garage?
The news garage doors are plugged into ordinary sockets, and so are drawing power permanently, but hopefully not much as unless the motor is running, it should only need a tiny bit to power the receiver. Other than that, the wiring was extended (much more than a year ago) from the garage on to the greenhouse (by a qualified sparks) where SWMBO runs a heater in the winter. I don't think that's activated yet as it's been so warm.
One oddity: in about August, a month after the doors were added, there was a thunderstorm and neither door worked after that. The sparks had added a new double socket for them, wired back to the unit in the photos. Only that new socket was no longer working, however, so I ran an extension to another socket so we could run the doors as normal until the sparks could get over here to take a look. All he did, he said afterwards, was replace the socket, which he had described as having "failed".
I should have asked him, failed in what way, but I didn't. Anyway it all worked after that until the recent spate of trips. I've been trying to get hold of him to do more work here (he made a nice job of redecorating the front room a year or so ago so we want more done) but he seems to be working all hours on some killer job in Bath and hasn't got back to us yet.
If I can get him to come over he should be able to do some leakage current tests to see where it's coming from.
Thanks for your comments.
With leakage trips the amount of power is not usually the issue... but things like dampness, damaged insulation, and other faults can play a part.
You could try unplugging one device for a while and see if that reduces or eliminates the trips.
(with lots of devices you can unplug half at a time to try and expedite the search!)
There is a section on the wiki on this:
Hard to make much of a guess about that - it could have been coincidence.
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