Erratic tripping

[leggedit]

Hello, this my first post and it is from South Africa.

Our house has a concumer unit fitted with 20a trips all of which had worked well.

Recently have changed lawn mower and this is now tripping the circuit to the outside socket immediately switched on.

Mower does not trip if plugged into any internal socket. Therefore assumed trip was faulty.

However rewiring external circuit through another trip switch also causes a trip.

Old mower (of lower power ) does not trip at all.

New mower works ok elsewhere and at other houses so have concluded there may be a wiring problem on the outside circuit which is a spur with only two sockets.

However this only shows as a fault with this particular mower and I am puzzled as to explanation.

I will be getting a local electrician to do full checks but I would appreciate any advice or comment and hopefully explanation before contact so I can determine if they know what they are doing......

Many thanks in anticipation.

 

[pushrod]

When you say "20A trips" do you mean mcbs or rcbos ? Or is there an rcd that is tripping?

 

[leggedit]

Sorry for delay in reply have been out all day.

i am not sure which is which. ie mcbs or rcbos ? Or is there an rcd that is tripping

Unit has Main switch 2pole then Earth leakage switch with test button the 2 no 30A trips then 10 no 20A trips for each of the circuits instead of fuses.

Trips made by CDI South Africa ref Hymag SA1G3 curve std 240v 50Hz if that helps.

I believe they operate in similar fashion to a conventional consumer unit.

If above does not help could you say what I should look for. Thanks

 

[jaresquire]

Hope you sort it, won't offer advice as I know South Africa does things different:-
A coworker of mine fresh from SA wired a fused spur for me, I went to connect equipment (admittedly with feed still live) and found out painfully that he'd switched the neutral, not the live..

 

[spsparks]

Are the other sockets you are using on a 20A trip too ? This sounds like an overload based on the info you have given

 

[pushrod]

RCDs detect earth faults (often have a little yellow test button). MCBs detect short circuits ( faults between live and neutral ). An rcbo does both functions. From your description it sounds like you have an rcd and then a number of mcbs. Some or possibly all of these are probably being protected by the rcd (your earth leakage switch).

Assuming it is an mcb that is tripping then your electrician should do insulation resistance tests between live and neutral on the suspect circuits. If the mcb itself is faulty these are usually tested/checked by swapping their position or replacing them. If it is the rcd that is tripping then they should do insulation resistance between the live + neutral to the earth. The rcd can also be tested to see what current is causing it ti trip and how quickly it is tripping.

hope this is of some help

 

[smartalek]

Did the other houses you tried the lawnmower with have rcd protection?(the earth leakage switch)? This will detect faults that mcbs (the 20a trips) won't.

 

[leggedit]

Thanks for replies will answer queries below

spsparks- other sockets all on similar 20A trips and mower works fine with them.

pushrod - I think you are correct that the number of mcbs are protected by the one rcd. However it is one of the mcbs that trips not the rcd and when I rewired the spur through another mcb that also tripped with the new mower and not the old so I have concluded it is not a faulty mcb.
If there is a problem with insulation between live and neutral on that spur (as seems likely as the main rcd is not tripping) then I am still puzzled why one mower works fine and the other does not.
Is it possible that a heavier current draw from one mower could trigger the mcb whilst a slighly lower current does not? It does not seem logical to me as any insulation problems would be apparent due to voltage potential according to my recolections of old O level physics...

smartalek - the other house did have rcd protection and there was no problem there at main or mcb level.

I very much appreciate your help and will almost certainly get a local contractor in to do some tests as suggested and will let you know what they find. It is particularly kind of you to try and answer my queries without seeing the problem first hand as I am sure you would quickly reach a solution were you here.

Any other comments will be welcomed ( and I promise not to mention the blue sky and 30 degree heat we are "suffering"...)

Many thanks again.

 

[pushrod]

from a distance it certainly is puzzling - have you tried putting something of a similar power (electric kettles are fairly high wattage) on to the external circuit and seeing if you can simulate the fault. It sounds like you have a short (between live and neutral) on that circuit that is only happening when it has a fairly high load/current through it. Could there be a chance that the cable has been damaged/nibbled by rodents? somewhere along its length. Getting insulation resistance tests done on the circuit would certainly seem to be the way to go.
BTW we love the cold , rain and biting wind...

 

[telectrix]

i hate the rain, cold and wind. plays havoc with my aging knee joints.

 

[pushrod]

i hate the rain, cold and wind. plays havoc with my aging knee joints.

Me too really, but i can pretend can't I?

 

[Marvo]

Hi Leggedit,

I see you're confusing the UK sparkies with the SA way of things.

A standard SA installation has a main 60Amp breaker (MCB) which is a green lever 2-pole. It's rated as an isolator (denoted by the green levers).

This supplies a 60Amp Earth Leakage breaker which may or may not offer overload protection. (RCD or RCBO). More commonly it's just RCD. The CBI Samite RCBO (with overload protection) which looks identical apart from the model number and it's about twice the price.

The earth leakage supplies a number of 20 Amp socket circuits. These would be classed as radials by UK standards and are run on 2.5mm FT+E cabling usually. The SA 0142 regs state that only socket outlets need to be on earth leakage protection, items such as light circuits, hot water cylinder and stove are optional as far as RCD is concerned.

Each socket circuit would usually have 4 x 16 Amp sockets in a domestic environment. These four sockets are often spread between high and low load areas. You might find one socket in the kitchen and the other three on the same circuit dotted around the dining room and lounge for example.

So...the outside socket would be on the same circuit as three other sockets probably. If the load on those other sockets is already quite high and you plug in the lawnmower, there's a good chance that the total load exceeds the 20Amps allowed by the breaker. The current of the lawn mower would be high especially when it's initially starting up.

Some things you can try;

1. Maybe start the lawnmower by pulling the trigger on-off-on-off at 1 second intervals untill it has picked up some speed, then keep it fully on.
2. Tilt the lawnmower whilst starting it so the cutters aren't trailing on the ground or in long grass.
3. You can try fitting a slower curve breaker which would be denoted by an orange toggle in the CBI Samite range. You're not allowed to exceed the 20Amp rating however unless you upgrade the entire socket circuit to larger cabling.
4. If none of these help then you'll need to supply your outside sockets on a separate dedicated circuit from your DB (consumer unit in the UK).

I would suggest you enlist the help of an electrician if you need to replace breakers or run new circuits. Look on the bright side, you should get a half decent sparkie with his mate at about R300.00 per hour, that's only about 33 quid.......a bargain if ever there was one

 

[pushrod]

Interesting read there Marvo- all makes sense now

 

[nickblake]

i hate the rain, cold and wind. plays havoc with my aging knee joints.

just those bits plays havok with every part of me ,ive taken to having a bath in WD40 its that bad lol

 

[leggedit]

In true African style the elctricians listened to my logic and then said it's an overload. Did not have the kit with him to check for insulation resistance.
I had already bought a 25A circuit breaker to try but it was a newer model and I was reluctant to force fit but he did anyway.
Result mower works fine problem solved but I am still unclear why the problem arose.
As pushrod says it seems the fault /short is only happening when there is a fairly high load on the circuit but my engineering brain refuses to accept that without proof. However in absenceof correct kit to test this I will just have to remain puzzled.
However mower works on original outside socket and the call out fee was reasonable.
Ie I got a fix but no explanation - that's Africa...
Many thanks for all your kind advice and suggestions.