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Topic Title: ELEX
Topic Summary: Sandown Park
Created On: 04 November 2012 06:51 PM
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 13 November 2012 09:06 AM
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kj scott

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Originally posted by: Zs

In passing, what do you do then, when changing a board and the new RCD won't hold and it is getting late? If essential, I put the offender on a non RCD breaker which I leave space for on old board changes, for the night, and go back the next day. I use the 'no less safe' phrase out loud to my clients. We, on here, always sort our RCD issues in the end but the time, caring etc. as above come into play on how we sort them.



Zs


Zs why do you not test before you start work, its quick, simple and saves you having to explain to the customer why everything worked before you touched it.

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 13 November 2012 09:31 AM
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Jobbo

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The Elecsa engineer (who was obviously more prepared for all the questions), did state that a mA clamp meter test should be used prior to the change of the consumer unit and explained how it should be done. It's something I've always done and although not 100% guaranteed (due to unknown items possibly not switched on at the time) it is good practice.
 13 November 2012 11:15 AM
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Zs

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Originally posted by: kj scott

Originally posted by: Zs



In passing, what do you do then, when changing a board and the new RCD won't hold and it is getting late? If essential, I put the offender on a non RCD breaker which I leave space for on old board changes, for the night, and go back the next day. I use the 'no less safe' phrase out loud to my clients. We, on here, always sort our RCD issues in the end but the time, caring etc. as above come into play on how we sort them.







Zs




Zs why do you not test before you start work, its quick, simple and saves you having to explain to the customer why everything worked before you touched it.


I have been poised for that one kj. Testing before, with a clamp meter, earth leakage and so on happens as part of the process but you show your innocence. Testing circuits is not going to tell us everything. But you give me the chance to tell a story.

Last summer I changed a board for a friend of JP, nice fella and an engineer. RCBOs. At the beginning of the day everything tested out beautifully. I had time to re earth the garage, re clip some cables and go the extra mile for him. At the end of the day, when everything was restored one of the RCBOs just would not hold. I was close to having to bypass the RCBO for him for the night. Apparently nothing had been plugged back in while I was there (?) but now I was getting bad results. It was an essential socket circuit.

Eventually I found the culprit, plugged in beside the bed, and strolled into the garden to ask him when he bought his alarm clock and who from? He told me he had had it a while and had changed the plug a while back for a nice new clean one. It was wired with N and E reversed. Oh how he begged me not to tell JP.

And what of poor test results at the beginning of the day? A board change becomes a major fault find which cannot be carried out in a day, and so on.

Zs
 13 November 2012 04:48 PM
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kj scott

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Still not with you there Zs, surely if your standing leakage is more than 15mA per circuit, or equivalent cumulative, further investigation is needed before installing a 30mA RCD/RCBO to the circuit.
The situation you describe would have been evident before works commenced; or quickly located using a leakage clamp meter.
Thanks for the assumed innocence, not so sure that I am though.

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 13 November 2012 05:16 PM
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AJJewsbury

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surely if your standing leakage is more than 15mA per circuit, or equivalent cumulative, further investigation is needed before installing a 30mA RCD/RCBO to the circuit.
The situation you describe would have been evident before works commenced;

I suspect Zs's point is that it's often appliances, rather than circuits, that are the cause of the problems - and unless they're plugged in/switched-on/calling for heat/at the right time of day or cycle/etc they'd be invisible to a test.

- Andy.
 13 November 2012 05:18 PM
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OMS

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I can think of many cases where detecting that would be a major task Keith - refrigeration compressors in run down mode or defrost cycles, trapped cables under floorboards or within back boxes, standing leakage of laods not always present, high protective conductor currents from "abnormal" kit

I'm sure you've seen a few yourself over the years

Even assuming you've found them all - often they can take quite a while to rectify - certainly outwith the time allowed to swap out a DB in an existing install

Even if identified prior to starting, the client still needs to arrange rectification.

I can easily see a case when bypassing an RCD would be pretty normal practice for an overnight period in a typical domestic DB swap in an occupied home

Regards

OMS

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Failure is always an option
 13 November 2012 06:47 PM
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kj scott

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Originally posted by: AJJewsbury

I suspect Zs's point is that it's often appliances, rather than circuits, that are the cause of the problems - and unless they're plugged in/switched-on/calling for heat/at the right time of day or cycle/etc they'd be invisible to a test.

- Andy.


But if you have establish a stable condition prior to commencing work, then an appliance creates a fault, it is easily solved, isolate it.

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 13 November 2012 06:53 PM
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kj scott

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Originally posted by: OMS

I can think of many cases where detecting that would be a major task Keith - refrigeration compressors in run down mode or defrost cycles, trapped cables under floorboards or within back boxes, standing leakage of laods not always present, high protective conductor currents from "abnormal" kit

I was not suggesting that Zs should undertake fault location, just not change the DB until the cause has been established.

I'm sure you've seen a few yourself over the years

Plenty.

Even assuming you've found them all - often they can take quite a while to rectify - certainly outwith the time allowed to swap out a DB in an existing install

Exactly, thats why you don't start in the first place, you advise the customer.

Even if identified prior to starting, the client still needs to arrange rectification.

Of course.

I can easily see a case when bypassing an RCD would be pretty normal practice for an overnight period in a typical domestic DB swap in an occupied home

Wasn't it the bypassing of an RCD that led to one of the first prosecutions under EAWR?

Regards



OMS


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 13 November 2012 07:29 PM
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OMS

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Wasn't it the bypassing of an RCD that led to one of the first prosecutions under EAWR?


No less safe than when you started ?

Regards

OMS

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 13 November 2012 07:49 PM
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mikejumper

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Originally posted by: Zoro
It is unfair that the NICEIC gets more stick than the other Schemes through just because they are larger.

Not unfair surely, just proportional.
 13 November 2012 09:14 PM
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kj scott

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Originally posted by: OMS

Wasn't it the bypassing of an RCD that led to one of the first prosecutions under EAWR?


No less safe than when you started ?

Regards

OMS


I wonder if the electrician, his manager or his director tried that one when they were prosecuted?

If they did it didn't work.

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 13 November 2012 10:09 PM
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AJJewsbury

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But if you have establish a stable condition prior to commencing work, then an appliance creates a fault, it is easily solved, isolate it.

I'm not sure it's necessarily that easy - I'm sure we've all been there ...

Householder: "That trip-switch thingy keeps tripping - any idea what's causing it? Usually it resets alright, but it's a pain having the electric going off all the time"
me: "OK what did you turn on when it happens"
Householder "Nothing - just watching the telly"
me: "Ah, OK, so what appliances did you have connected at the time?"
Householder "Well, the TV (obviously), Sky box, the Mrs had the iron on, and a table lamp in the front room"
me: "anything else?"
Householder: "no, don't think so"
me "what about the: Cooker, Fridge, Central Heating, TV aerial amplifier, PC, mobile phone charger, kids playstation, WiFi router, alarm system, microwave, .."
Householder: "Oh, yeah, now you mention it..."

and then eventually it turns out to the one particular phase of the washing machine cycle, the outside light on a PIR when it's been raining in the previous week or a nail through a loose board just touching a cable that only makes contact when the cat sits in a particular corner.

- Andy.
 14 November 2012 06:38 AM
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kj scott

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314.1
This is where a leakage clamp meter comes into its own as fault finding instrument. You can quickly eliminate the potential sources of earth current.
Obviously individual program switching faults will be more difficult to locate.
Are you responsible for all of the customers current and future appliance use; or the conformance of the fixed installation?

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Edited: 14 November 2012 at 06:54 AM by kj scott
 14 November 2012 10:55 AM
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John Peckham

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"No less safe than when you started" = " Just as dangerous as when I started".


RCDs get a very bad press. 99.99% of the time they are just doing their job. What is called for is a bit of engineering judgement which most persons engaged in electrical installation do not have as they lack a basic knowledge of electrical principles.

The advice given by the electrical experts at Elex was at best irresponsible as a fair proportion of the audience will accept it as gospel that it is acceptable to connect a faulty circuit to the Non RCD of a board as good practice. They will be off to the wholesalers to tell their fellow Sun readers that it is OK to do this as the ESC, NICEIC, ELECSA and NAPIT all say it is OK. Thus a potentially dangerous practice will pass in to folk law to be promulgated and passed down for several generations as if written on tablets of stone.

Lights blue touch paper and retires!

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John Peckham

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 14 November 2012 11:17 AM
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OMS

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"No less safe than when you started" = " Just as dangerous as when I started".


Mmmm - I suspect that's a bit black and white John, and probably incorrect from a legal perspective anyway.

For sure, some will want to reduce thier actions in any given circumstances to a simple check list type approach, but generally, most sparks can determine what is truly dangerous compared to what is potentially dangerous given an extended fault sequence and most often just nuisance tripping.

Is there anything actuall that "wrong" with discovering a problem that pulls out a 30mA RCD and leaving said circuit as was until further investigation/remediation occurs if the person with direct observation of the problem deems it sufficiently safe.

Just for clarity - we are talking about an existing non RCD protected circuit being a bit of a nuisance when being offerd RCD protection rather than a circuit with an existing RCD being bypassed

regards

OMS

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 14 November 2012 05:26 PM
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John Peckham

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"Mmmm - I suspect that's a bit black and white John, and probably incorrect from a legal perspective anyway. "


Exactly my point you need to exercise a big helping of engineering judgement to re-connect a circuit that is popping a 30mA RCD even as a very short term temporary measure. My colleagues went away with the impression that the audience was being given the green light to connect faulty circuits to the non-RCD side of a board as a long term fix as an alternative to fault finding.

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John Peckham

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 14 November 2012 05:51 PM
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OMS

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Ahhh OK John - then clearly the advice is "misguided" shall we say

Regards

OMS

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 14 November 2012 08:43 PM
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Zs

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Originally posted by: kj scott

314.1

This is where a leakage clamp meter comes into its own as fault finding instrument. You can quickly eliminate the potential sources of earth current.



I use one all the time. It is my primary piece of RCD problem sorting magic and is used at the beginning of a DB change day.

But it does not quickly eliminate the potential sources of earth current. You need instinct for that. That trip is usually a quick flash of excess earth leakage sending it over the edge. Easy to miss if you blink or are still on your way back down the stairs.

Not least, getting an earth leakage clamp meter around the line and the neutral together is the only way of measuring earth leakage properly and there are times when it is not possible to do that.

Most homes have at least 8mA of Earth leakage in normal usage, and many have 15-19mA. Your average RCD will go over the edge at about 28mA and a device switching or refreshing can do that in the blink of an eye.

Zs
 15 November 2012 08:46 AM
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kj scott

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Originally posted by: Zs

Originally posted by: kj scott

314.1

This is where a leakage clamp meter comes into its own as fault finding instrument. You can quickly eliminate the potential sources of earth current.




I use one all the time. It is my primary piece of RCD problem sorting magic and is used at the beginning of a DB change day.

So you know and agree its value.

But it does not quickly eliminate the potential sources of earth current. You need instinct for that. That trip is usually a quick flash of excess earth leakage sending it over the edge. Easy to miss if you blink or are still on your way back down the stairs.


I find that it is very quick when coupled with experience and engineering judgement.


Not least, getting an earth leakage clamp meter around the line and the neutral together is the only way of measuring earth leakage properly and there are times when it is not possible to do that.

That can be a problem at times with the smaller clamps, but if required there are larger ones and flexible coils.

Most homes have at least 8mA of Earth leakage in normal usage, and many have 15-19mA. Your average RCD will go over the edge at about 28mA and a device switching or refreshing can do that in the blink of an eye.

RCDs can trip at significantly lower residual currents than 28mA, this value is common to MEM devices. I would not put a 30 mA RCD/RCBO on a circuit displaying 15-19mA. This is why we have 314.1; and also test RCDs at half their rated value

Zs


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IET » Wiring and the regulations » ELEX

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