Tripping RCD by neighbour's mower

[phildam]

RCD trips when neighbour mows his lawn.
We have just had an electrician round to check our electrics and he can find no problems. He performed several tests of which the following were his notes/results:

We estimate that the MK Sentry Consumer Unit is around about 15 years old.

TNCS System
External earth fault loop impedance
Ze = 0.15
Prospective short circuit current
1.5KA
RCD Trip 18MS
House Trip 17MS
Therefore RCD is GOOD

So he went to the meter box and started to pull in and out a few times a fuse, this tripped the RCD after about 2 attempts. Every time the RCD was reset it did it again. He repeated this about 4 times and the RCD tripped each time. He suspect that the RCD needs replacing.

Then the neighbours arrived home and decided to cut the grass. The electrician looked on with interest. The lawn mover was plugged in the garage as usual as that is where he keeps the mower. Would you adam and eve it? It did not trip….. However, after a couple of minutes mowing the plug in the mower came out and hey presto he tripped my RCD.

The electricians had a look at the neighbours fuse box and informed him that for his own safety he should get it changed as it is the old type that does not have an RCD. It was the one that was installed when the house was built about 23 years ago. At the very least he should get a plug-in RCD for his mower.

The electrician then went on to say that when he was playing with the fuse in my meter cupboard, he was simulating the mower tripping the RCD and suggests we get the RCD replaced at a cost of about £130 inclusive of fitting and vat.

Is this really necessary to stop the tripping of our RCD or could it be something else that needs to be addressed. And how do we stand with regards the neighbours not having an RCD.

Please help ! ! ! ! !

 

[Adam_151]

From what I gather a replacemnt RCD may well solve it, RCD technology has advanced quite a lot since 15 years ago AFAIK, and I know of people reporting similar problems to this with wylex RCDs from the '80s etc

The price he quoted does seem a little steep tbh, but I also believe MK have changed the design, so I'm not sure he'll get a more modern one to fit, unless that price is to swap out the CU in which case its a good price (worryingly good... as in you wonder where the corners are being cut)

not sure what the two readings are, you should have one at 1x which should be sub 200ms, and one at 5x which should be sub 40ms, also a 0.5x which should result in no trip. I'm guessing rcd trip is 1x and house trip is 5x . A ramp test might have been usful, but its not a required test and his meter might not have been able to do it

Pulling the fuse in and out is hardly scientific, and I hope you didn't have any computers on that circuit , and I guess he is trying to see if slight flucuations in the supply from a load being applied affect the RCD, I guess that as it did, it links in with my first point

The fact that the neighbour has no RCD is of no consequence to you, the neighbour will be in greater safety if he has one fitted, but its not related to this problem

 

[phildam]

The guy said that to repalce a "MK RCD LN5780s 80A 30 mA" unit was £130 inclusive of fitting and VAT. Is this a good price or a rip off? Is there a better alternative that would resolve this problem?

 

[Adam_151]

The RCD itself will probably cost between £35 and £45, and I shouldn't think it'd take more than an hour to fit and test, but then you have got the fact that its a little job, and it buggers up the day and all the travelling to do very little? not sure how it pans out in the end, my gut reaction is its a little steep, but it may well not be all things considered.

And I'd be concerned that if your CU is 15 years old, the new RCD might not fit... can you take and post a photo, I know the older MK units need a special kit to fit modern MCBs, and I beleive RCDs have changed shape as well, but unfortunatly no magic converter exists for them

 

[the_jinj]

Why not get another quote/opinion from a local electrician if you think he's ripping you off/wrong

 

[cozycats]

most trades around here price anything by the half day ie whatever is going to be £75 + parts and vat. I would think £130 not unreasonable!

 

[JohnD]

The LN5780 is a 4-module Sentry unit (same size as a TPN RCD) which is bigger than is fitted to modern domestic CUs. I see it was fitted to split-load CUs like the 5686. If it is still made it may not be updated electronic technology, though. The "s" suffix on your part no means something but I can't remember what.

Back in 1990 MK were selling the 2-module DP RCDs but not up to the 80A capacity.

As you are TNCS I presume this is a split-load? If it was whole-house it might be less trouble to fit a more modern device in its own enclosure as a main switch.

There were also SP RCBOs of this vintage which might still be available, in my view a better solution than split-load. p/n 6330 is 32A 30mA.

MK of this period were top quality, though RCDs don't seem to last forever. Try to get some kind of assurance that changing the RCD will solve your problem, though.

 

[Jim2287]

he also has to certify i think, and that costs money

 

[Jaymack]

The guy said that to repalce a "MK RCD LN5780s 80A 30 mA"

The MK site doesn't list this item number, is it a voltage operated ELCB?, (these are no longer acceptable and may be a reason for your problems). Can you post all the details on the existing unit or post a photo? if a voltage operated ELCB, there should a tail connected to earth.

Jaymack

 

[Jaymack]

The "s" suffix on your part no means something but I can't remember what.

S = Time delay.

Jaymack

 

[JohnD]

S = Time delay.

Jaymack

Oh yes!

Odd to have it on a 30mA RCD though - I thought time delays were usually 100mA or greater? And usually fitted if there is a 30ms RCD on the same circuit. Time delays s/b fairly immune from nuisance tripping. It would be interesting to see a photo of the installation.

I have a 1990 MK product catalogue, it shows p/n 5780 as current operated (not voltage) but does not show 5780s

And:

RCD Trip 18MS
House Trip 17MS
Therefore RCD is GOOD

so I don't believe the current RCD can be delayed. Perhaps the electrician is recommending one to avoid nuisance tripping, sounds a fair start, but what about the downstairs sockets?

 

[Adam_151]

All MK part numbers for consumer unit bits seem to end in S, I belive it stands for sentry

5500s for example is a 100A DP isolator
5945s is a B45 breaker
etc

5780s according to my MK product PDF is a bog standard 80A 30ma two module RCD

 

[JohnD]

All MK part numbers for consumer unit bits seem to end in S, I belive it stands for sentry

5500s for example is a 100A DP isolator
5945s is a B45 breaker
etc

5780s according to my MK product PDF is a bog standard 80A 30ma two module RCD

Aha I bet the "s" is for the second version of Sentry. The current version is not quite the same as the 1990 version, and the MCBs are not interchangable though you can buy a little busbar converter.