Never seen this before..!!

[apprentice87]

Hi all,

What you make of this??

Overhead TT SP supply

Just about to go out yesterday, when the village idiot turns up moaning that he used something in his garage [remember this] and now all his electrics have tripped off.

I ask, the MCB or an RCD??. He says "both" and that neither will reset...

Ok, so i tell him that, what if you leave the offending MCB off, and then reset the RCD. He says that the RCD will still not reset...

Ok i say, you have got a neutral/earth fault that could be on ANY of the circuits...

I go up there with nothing but a screwdriver i happened to have with me, no means of checking for dead, measuring voltages, or a "neon" or anything, so i was a bit limited in what i could do.

It is an old plastic Mk CU set up as a split load affair. No main switch, the RCD's used as a main switch [Yeah, brilliant, especially on TT] and also, another thing i cannot understand, is why they have got the two RCD's in series, one, [a 100ma TD one] acting as a front end RCD [and also protecting the lighting circuits??] and then a 30mA one protecting the sockets.

Why would you want to do a thing like that?? Myself, i would have a proper DP main switch, and then two 30mA RCD's. No need for a "front end" one, and no need for a 100mA TD one either, but there you go..

Anyway, Here is what i do..

First off, an RCD works by comparing blah blah, you know what i mean, and also, so far as i can remember, the things need more or less mains voltage to operate their own "internals"

So, i switch off the offending MCB. The RCD will not reset.. I take the cable out of the MCB [just in case it was not working] RCD still will not reset..

Ok, so i disconnect the neutral from that circuit. MCB resets.. So, Yehhhh!! was right all along. neutral earth fault on that circuit.. the 100 yard long one feeding his garage.

Now, not wishing to be killed, in between touching anything, i pulled the fuse. AND, as i am not daft and fully aware that the neutral can be very "live" too!! [for various reasons...] i proceeded very carefully.....

Now, wait for this one..... If i disconnected the circuit in questions neutral from the neutral bar, the RCD would reset. The second i went to replace it, [as a test] the RCD tripped, BUT, with the main fuse STILL OUT???

How on earth can an RCD operate when the fuse is pulled.. My opinion is that his installation must have some considerable PD between earth and neutral.. not good.

I did wonder if the cable to the garage [a SWA jobbie] was connected to earth in anyway at the far end, [and hence one possible source of differing potentials] but no, it was not it is terminated in an adaptable box that is waving in fresh air..

Any thoughts?? Everything works normally, so i would think that there is the usual 230 between phase and neutral, so i doubt a DNO neutral fault, so the other idea was that something is livening up his earth and therefore giving a large PD from that on the neutral..

This bloke will NOT spend anything on the house or electrics, and, as i left, he was moaning about "****ty RCD's"

I did point out that if it were not for his one, he would now be dead. Get this!! He had tried to experiment to fix it himself, by digging up his earth rod! as he thought this might stop the RCD tripping....

john...

 

[steptoe]

First up,

100mA TD upfront on a TT was a totally standard method in 15th (16th also sometimes) with no main switch, lights etc only required to be on 100mA

I'll read rest when I get a chance.

 

[Lurch]

It is an old plastic Mk CU set up as a split load affair. No main switch, the RCD's used as a main switch [Yeah, brilliant, especially on TT] and also, another thing i cannot understand, is why they have got the two RCD's in series, one, [a 100ma TD one] acting as a front end RCD [and also protecting the lighting circuits??] and then a 30mA one protecting the sockets.

Why would you want to do a thing like that?? Myself, i would have a proper DP main switch, and then two 30mA RCD's. No need for a "front end" one, and no need for a 100mA TD one either, but there you go..

Erm, totally fine and I've installed maybe hundreds of boards like this. Pretty standard TT time delay split load.

If you think for a minute you will realise this is a 16th edition split load but because it is TT there was a requirement for all circuits to be on a 100mA TD RCD becasue of the Zs so the main switch is a 100mA TD RCD so all non 30mA circuits get RCD protection.

As for the rest it, you should know better than to turn up here with some half arsed story with no idea what is happening. First job, get your test gear out. My crystal ball is in for a waxing today.

I'll read rest when I get a chance.

I wouldn't bother.

 

[steptoe]

I've actually got a 100mA TD upfront in my house, and its PME, proper real PME at that,

 

[ProDave]

Go back with some test gear and tell us what you find.

An rcd will trip if it senses an inballance, so obviously you have something passing through the neutral, even though nothing is connected to live (fuse out)  My guess is neutral is not earthed at substation, or there is a significant difference in local earth an substation earth potential.

This all started when he plugged something in in the garage. No mention that he has actually unplugged that something yet?

 

[steptoe]

I don't have a picture since I tidied it up a bit.

 

[Doc Hudson]

Current on site guide, section 3.6.3 discusses application of RCDs, Figure 3.6.3(ii) gives an illustration of typical split board with 100ma time delayed s-type RCD used as main switch, and a 30ma RCD protecting part of the board.

Doc H. 

 

[apprentice87]

First up,

100mA TD upfront on a TT was a totally standard method in 15th (16th also sometimes) with no main switch, lights etc only required to be on 100mA

I'll read rest when I get a chance.

Hi Steps, yes that might well have been the case, but what i was getting at, is, the way i see it, in this gentlemans installation, there is absolutely nothing to be gained by the up front RCD. [and lots to be lost..]

True, as it protects the lights and only the lights, it will discriminate with the 30mA one so the lights stay on.. Now, i know that things do not have to comply with the latest version of the regs, but this setup would not be allowed now anyway, unless he wants to re-wire his lights in SWA that is..

Knowing that RCD's are about the most unreliable thing there is, i would think that there would be a lot to be gained, by simply swapping the 100mA TD one for a 30mA none time delay one. This will not only give protection to the wiring in the walls [not being in earthed conduit, 50mm blah blah] and, even better, would act as "redundant protection" too [being in series with the second one] and hence give much more reliable ADS for the sockets in the event of a fault..

That is the way i see it anyway...

john..

Current on site guide, section 3.6.3 discusses application of RCDs, Figure 3.6.3(ii) gives an illustration of typical split board with 100ma time delayed s-type RCD used as main switch, and a 30ma RCD protecting part of the board.

Doc H. 

It might do, but what does it ALSO say about the wiring for the circuits protected by the 100mA TD...

john..

 

[apprentice87]

Go back with some test gear and tell us what you find.

An rcd will trip if it senses an inballance, so obviously you have something passing through the neutral, even though nothing is connected to live (fuse out)  My guess is neutral is not earthed at substation, or there is a significant difference in local earth an substation earth potential.

This all started when he plugged something in in the garage. No mention that he has actually unplugged that something yet?

Hi Dave, I did tell him that there was something SERIOUSLY not right and that he might like to contact the DNO, but he does not want to!!

The circuit with the fault i repaired last night, was a neutral shorted to earth by being cut where it passes through a gland [it was swa] I did tell him that SWA was not intended to be used as "flex" but you cannot tell some folks. With him constantly coiling and uncoiling it the conductors had pulled back into the cable and wedged themselves and a connector block into the end of the gland!!

I will pop up later and have a measure of the various voltages and see what i find..

john..

 

[steptoe]

@apprentice87

John, you won't simply be able to swap out the 100 for a 30, the board will be configured wrong,

The 30 at present is (should be) off the load side of the 100,

You could (possibly) take it off the feed side, and add an external REC2 , afaiaa, there is no reg states the main switch must be in the same enclosure,

BUT, you are modifying a type approved enclosure, which is possibly a no no,

And any other enclosure you fit must be amd3 compliant, which also rules out a standard REC2, 

Tbh, I'm failing to find the problem with what is already there,

Its like saying @ProDave needs to have an airbag and retractable seatbelts in his Landy.

EDIT

Would adding another 30mA in series actually be making the install worse, in terms of being inconvenience under fault conditions, :C

 

[Lurch]

Knowing that RCD's are about the most unreliable thing there is, i would think that there would be a lot to be gained, by simply swapping the 100mA TD one for a 30mA none time delay one. This will not only give protection to the wiring in the walls [not being in earthed conduit, 50mm blah blah] and, even better, would act as "redundant protection" too [being in series with the second one] and hence give much more reliable ADS for the sockets in the event of a fault..

Have you been drinking?

Sometimes you come out with some reasonably sensible stuff John but today you are just spouting complete nonsensical gibberish.

 

[apprentice87]

EDIT

Would adding another 30mA in series actually be making the install worse, in terms of being inconvenience under fault conditions, :C

Well, yes,,, but i wrong rather be inconvenienced then dead!!

Its like saying @ProDave needs to have an airbag and retractable seatbelts in his Landy

Ha ha!!! Nearly peed myself reading that!!

john..

Have you been drinking?

Sometimes you come out with some reasonably sensible stuff John but today you are just spouting complete nonsensical gibberish.

So you think the idea of "redundant protection" is nonsensical gibberish??

john..

 

[Lurch]

So you think the idea of "redundant protection" is nonsensical gibberish??

I don't even know what to say to this.

 

[binky]

RCDs reliable???

Just swapped out a 30mA Hager RCBO in favour of  Control Gear 100mA Type S on a set of solar panels I fitted 2 year ago on farm shed roof. Customer was complaining of occasional tripping in damp conditions. RCBO has been working fine for months, RCD tripped out every 20 mins (mind you it has been knocking around back of van for about a year). Can't find anything wrong with circuitry, and inverter reckons max insulation figures on array (self tests on start-up). The inverter is ABB which are a bit famous for earth leakage, but still don't understand why RCD 3 times less sensitive keeps tripping.

 

[apprentice87]

Hi Binky,

RCD's are indeed lettuced!! Wonderful idea, but why are they so lettuced and unreliable?? That is why i would have two in series. Downside none, upside, might save someone from being killed..

john..

 

[binky]

if the customer wasn't such a tight wad, I would replace MCBs with RCBOs and leave 100ma as main switch

 

[apprentice87]

Sounds good to me!

john..