Saftey before RCDs

[marcuswareham]

I understand the importance of RCDs, what they do & how they can save lives. I agree with there requirements. I also understand you can meet ADS disconnection times with high enough Zs values for relevant fuses or MCBs, etc

In the 18th edition the use of RCDs is much more widespread

The question is RCDs are not new and have been around for some time, In the 15th edition in 1981 RCD use was not so widespread and as far as I can tell (I don’t have a copy and was not alive when they came out) it was only required for a socket which could maybe be used outside to be RCD protected, however the risks were much the same in 1981 we still use a lot of the same appliances used then (more computers now) but things like power tools which could cut there own power cable was used for example , etc

So what changed why was it safe then but not now?

I guess this argument could be taken back to the 1st edition of the wiring regs which now look like a joke, although our usage and power requirements have changed dramatically since then hence the need for updates

Maybe this is a silly question, like why do we now need smartphones but didn’t 20 years ago, but would be interested in peoples views

Thanks Marcus

 

[derek]

My understanding is that in the past there was more of a reliance of the Faraday Cage idea. This wouldn't extend outside the house so then RCD protection was required as far as I know.
I only learnt from the 17th edition though, I do recall there was a lot of bonding of metal window frames and such, at least at my grandparents house, I'd be interested to know if that was a requirement of earlier editions of the regs or just a misinterpretation.
Zs values should be low as possible I assume you meant.
There were voltage operated earth leakage detection devices before apparently, but I haven't seen one installed yet, and don't know much about them except they are generally considered to be inferior to RCDs.

 

[cliffed]

Rcd were used widely in European countries before we adopted the format.
Bonding of anything metal,back in the 14th/15th,including windows,external waste pipes,baths,radiators in bathrooms,those are Some of the items.
The Rcd was used mainly in the domestic environment,& provided added protection,against electric shock.
The bonding also disappeared,on the above,if protected by Rcd.
Bonding to water & gas still needs to be done,though the 18th ,says if a plastic insert enters the building,then metallic pipework then no bonding required.
Still can’t get myself to do that,I always install a cable @ those positions.

 

[littlespark]

Cost would be a considering factor.
RCDs would have been a new or developing technology, in the times when there was nothing miniature about miniature circuit breakers.
If an rcd was fitted, it was as a whole house mainswitch, or even upfront... so as not go to the cost of replacing the whole board.
Then it became split boards, a little separation in the event of a fault.
It is only in recent years that a rcbo can be used for every circuit in a board. Down to cost of the individual device and the space it can take up.

Now we are being asked to use SPD and consider AFDD in the boards... new, expensive but as some point will be commonplace

 

[derek]

I have worked in some old property refurbishments and a substantial number had metallic conduit electrical installations buried in the building fabric which obviously provides pretty good protection from a DIYer. Dodgy appliances with poor earthing etc. could still have been a risk.
I seem to recall being told at college that the regs were originally written to cut down on the numbers of electrical fires, and have over the years adapted to cover user safety more comprehensively.

 

[marcuswareham]

Zs values should be low as possible I assume you meant.

Yes low enough sorry, In my head I was thinking of PFC being high enough

 

[pc1966]

Part of the move to more RCD use comes from increasing expectations of safety (along with more DIY accidents) together with the falling price of electronics and (as already mentioned) the fact they are physically smaller now.

The Europeans have had incomer RCD for a long time, but generally they have used TT systems so they need that to disconnect on a fault. Whereas the UK with its long standing use of TN-S (and more recently TN-C-S) and polarised sockets with earth pins has been able to rely on a low Zs clearing faults to earth in equipment or bonded metalwork by the fuse/MCB tripping.

 

[marcuswareham]

Thanks for all the replies, interesting stuff

I understand about the faraday gage concept and had no idea metal window frames used to be bonded, there must have been bonding cables everywhere!

Yes I thought cost might be a factor and what cost was reasonable for an installation

My partner's dads house has one of the old voltage style earth leakage devices, it is a TT supply so I guess they were used before RCDs due to the high earth electrode resistance. His place has since had the C/U replaced but the VOELCB has been left in place before the c/u. He said he has had RCDs trip and the VOELCB does nothing.

I see the point that there was less risk from drilling though cables it with earthed metal conduit in the wall, which was commonplace, I guess that also stopped being so common due to cost. Even in my house though which was 16th edition and had no RCD protection on over half of circuits the cables are chased in and not very well mechanically protected. My house now is fully RCD protected with RCBOs

Has there been an increase in people getting killed or injured from electric shock then, compared to the past (but could this be down to a larger population)

 

[pc1966]

Now we are being asked to use SPD and consider AFDD in the boards... new, expensive but as some point will be commonplace

The rise of SPD comes from the big cost of electronics in the home now, cost for it is not much in terms of a CU change.

For now AFDD are expensive, a typical CU change with them is going to cost £1-2k more, but in time they will fall in price and probable will become the norm, a bit like RCBO are now small enough and cheap enough to consider using in place of a split-RCD/MCB board.
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His place has since had the C/U replaced but the VOELCB has been left in place before the c/u. He said he has had RCDs trip and the VOELCB does nothing.

There are two possible explanations for that!

Realistically if he had a 100mA/300mA S-type up front (and 30mA per circuit) it would not have problem trips either, but if you have a 30mA device for a whole house it is not going to be reliable!

 

[Mark Wright]

@OP It's still relevant to some extent because there's still a number of property's that don't have RCD's and some probably still just on Fuses.

as a side note a lot of not needed Bonding took place.

and the old ELCB..... but that's another story.....

and Safe Zones.

 

[marcuswareham]

£1-2k wow!!

Realistically if he had a 100mA/300mA S-type up front (and 30mA per circuit) it would not have problem trips either, but if you have a 30mA device for a whole house it is not going to be reliable!

The discrimination at his place doesn't work, there is no 100mA or time-delayed units everything is 30mA, not sure who did the c/u change etc, but it was not very thought out

 

[pc1966]

@OP It's still relevant to some extent because there's still a number of property's that don't have RCD's and some probably still just on Fuses.

But remember that an RCD provides additional protection and one aspect that bothers me going forward is a move to rely on it and drop the expectations of fault clearing Zs and bonding of exposed metal parts, etc.

RCD do fail, the electronics within are more complex than a MCB, and most home owners do not test them!
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£1-2k wow!!

The Wylex ones are neatest, fit single slot, but cost here is £154+VAT each:

Wylex NXSB10AFDType B AFDD Sw Neutral RCBO 1 Module Wide 10A 30mA - Shop4 Electrical

Buy our Wylex NXSB10AFDType B AFDD Sw Neutral RCBO 1 Module Wide 10A 30mA. Shop now.

www.discount-electrical.co.uk

So if you have 10 circuits in a CU that adds up!

The discrimination at his place doesn't work, there is no 100mA or time-delayed units everything is 30mA, not sure who did the c/u change etc, but it was not very thought out

Whoever did it was clearly not that competent! (To be fair, they might be technically skilled to fit it safely but just lacking in the theoretical stuff)

 

[marcuswareham]

@OP It's still relevant to some extent because there's still a number of property's that don't have RCD's and some probably still just on Fuses.

as a side note a lot of not needed Bonding took place.

Yes exactly still relevant, A friend of mine has just bought a house with a re-wirable fuse board.

He asked me if the plug in style MCBs would make it compliant with the current standards (he has not got alot of electrical knowledge). I said well the regs are not retrospective so there is not a requirement to upgrade and the plug in MCBs do the same job as the fuses/fuse wire I said either save your money and buy a pack of fuse wire, or spend your money on adding RCD protection (c/u change etc)

 

[Mark Wright]

But remember that an RCD provides additional protection and one aspect that bothers me going forward is a move to rely on it and drop the expectations of fault clearing Zs and bonding of exposed metal parts, etc.

RCD do fail, the electronics within are more complex than a MCB, and most home owners do not test them!

Yeah it's still relevant but looking to the future maybe some kind of Programmable RCBO with LCD screen that you can monitor the Earth Voltage/Current or maybe I'm dreaming.......

 

[pc1966]

Yes exactly still relevant, A friend of mine has just bought a house with a re-wirable fuse board.

He asked me if the plug in style MCBs would make it compliant with the current standards (he has not got alot of electrical knowledge). I said well the regs are not retrospective so there is not a requirement to upgrade and the plug in MCBs do the same job as the fuses/fuse wire I said either save your money and buy a pack of fuse wire, or spend your money on adding RCD protection (c/u change etc)

The plug-in MCB are a great stop-gap solution if a new CU is too expensive.

The majority of the public struggles to replace fuse wire competently, often putting in the wrong thickness or messing up the screws. Not to mention it is usually the light circuit that blows then you can't see to do it! Also it is obvious which circuit has tripped without having to rely on the documentation (which is frequently missing).

 

[marcuswareham]

The plug-in MCB are a great stop-gap solution if a new CU is too expensive.

The majority of the public struggles to replace fuse wire competently, often putting in the wrong thickness or messing up the screws. Not to mention it is usually the light circuit that blows then you can't see to do it! Also it is obvious which circuit has tripped without having to rely on the documentation (which is frequently missing).

Good point, didn't think about having to do it in the dark etc, and as you say changing fuse wire is simple enough to us but maybe not everyone

I guess the plug in MCBs also stop people putting in higher rated wire to avoid tripping!!!

I think he is going to go with a c/u change and more work any way as he is planning an exstension and new kitchen etc, so would make sense

 

[Andy78]

Yeah it's still relevant but looking to the future maybe some kind of Programmable RCBO with LCD screen that you can monitor the Earth Voltage/Current or maybe I'm dreaming.......

I'm sure digital programmable circuit breakers are already a thing.

 

[pc1966]

Good point, didn't think about having to do it in the dark etc, and as you say changing fuse wire is simple enough to us but maybe not everyone

I guess the plug in MCBs also stop people putting in higher rated wire to avoid tripping!!!

Also importantly they also stop people plugging in an open fuse on to a live fault. That rarely ends well

I think he is going to go with a c/u change and more work any way as he is planning an extension and new kitchen etc, so would make sense

Most kitchen fitters would insist on electrics complying with current regs so RCDs, etc.

The place may not need rewiring, an inspection and some insulation testing should confirm that, but going for a new RCBO board is what most of us would recommend anyway and is not a huge cost impact on a kitchen project, etc, probably costing £5k or more!

 

[davesparks]

I'm sure digital programmable circuit breakers are already a thing.

Yes they are, but I've only seen MCCBs with this functionality rather than MCBs

Have a look at the microcontroller tripping units available for schneider MCCBs if you are interested, I think they are called micrologix or something like that.

 

[Andy78]

This is what I remembered seeing and was thinking of.

Atom Power Is Launching the Era of Digital Circuit Breakers

New digital circuit breakers that combine computing power with wireless connectivity may finally replace an 140-year-old electromechanical technology

spectrum.ieee.org

 

[davesparks]

This is what I remembered seeing and was thinking of.

Atom Power Is Launching the Era of Digital Circuit Breakers

New digital circuit breakers that combine computing power with wireless connectivity may finally replace an 140-year-old electromechanical technology

spectrum.ieee.org

I don't like the look of those, I can understand the electronically controlled detection/tripping units operating a mechanicaly switching MCCB like the schneider ones I mentioned, but I don't like the idea of electronic switching at all.

You'd surely have to have an additional isolator for each circuit to be able to isolate it to work on it, and probably have to back them up with fuses.

 

[pc1966]

I have also seen MCCB with adjustable parameters, both over-current and RCD, but never had to deploy one myself.

As for those new Atom Power breakers I am sceptical. It said:

Atom Power’s digital breaker works 3,000 times faster. It is essentially a large silicon carbide transistor circuit that measures load current and switches the transistor off when the current level is dangerous.

For a start the SiC device will have higher pass impedance than mechanical contacts, so likely dissipating a whole lot more heat. Secondly I doubt it would be approved here for isolation / safety interruption as no mention of physical separation. Under heavy fault conditions is there a back-up fuse in case the SiC device fails short? Finally while the remote control has its attractions, it is also a huge security hole if someone half way round the would can switch stuff on/off, or even just adjust trip levels.

My background is electronics, I have designed hardware and written software for control systems and always went for simple systems for fault protection that could be 100% tested. I will take a lot of persuasion!

 

[davesparks]

Atom Power’s digital breaker works 3,000 times faster. It is essentially a large silicon carbide transistor circuit that measures load current and switches the transistor off when the current level is dangerous.

Do we need circuit breakers that are 3000 times faster?
The circuit breakers we have achieve the required 0.4 or 5 second disconnections perfectly well.

From what I've seen a lot of electricians struggle to understand setting an MCCB whne there are only 2 or maybe 3 settings to adjust, I doubt many will take the time to learn how to set up a whole adjustable tripping curve or whatever else these things will have. I suspect the vast majority would be set to 'default' and never adjusted.

 

[pc1966]

Do we need circuit breakers that are 3000 times faster?
The circuit breakers we have achieve the required 0.4 or 5 second disconnections perfectly well.

The only case where you might want to disconnect faster than the ~5ms magnetic trip is for fault current limiting, which HRC fuses generally do well, but breakers struggle to achieve.

America has a big thing about arc-flash, there are a lot of injuries and deaths (comparatively speaking) related to that compared to here. I don't know why, maybe more high power machinery, maybe differences in electrical system design, etc.

In those cases limiting the total arc energy reduces the damage that burns/UV will do the the unfortunate observer, and fast disconnection helps achieve that. But in many cases that really is only an issue for seriously big systems and not domestic or light commercial stuff. There are already systems that help control it such as differential relays that spot an internal fault (to act fast instead of waiting for downstream breakers to clear which they wont), some that have optical sensors to "see" the arc in a busbar chamber, switchgear, etc, and react instantly.

So I am not convinced this is solving a problem that is not already pretty much handled in any well-designed system.

From what I've seen a lot of electricians struggle to understand setting an MCCB when there are only 2 or maybe 3 settings to adjust, I doubt many will take the time to learn how to set up a whole adjustable tripping curve or whatever else these things will have. I suspect the vast majority would be set to 'default' and never adjusted.

Yes, and no doubt it will be crappy software that is unsupported in 5-10 year forcing you to upgrade...

 

[Mark Wright]

I will just ask while we're roughly in the area and saves me starting a new thread.

VOELCB sometimes you see these just been left as a main isolating switch and no earth connected anymore but is there any harm in leaving these connected up for what they were intended? they work differently from RCD and work on Voltage require quite a high voltage I think(i don't know?) and built like tanks, they just seem like another line of protection?

the principal of monitoring the Earth seems like a good idea but they don't seem to have been very popular.

 

[pc1966]

VOELCB sometimes you see these just been left as a main isolating switch and no earth connected anymore but is there any harm in leaving these connected up for what they were intended? they work differently from RCD and work on Voltage require quite a high voltage I think(i don't know?) and built like tanks, they just seem like another line of protection?

If there is no (usable) earth connected they will not work! That is a fundamental difference compared to the modern style of RCD that generally only needs L-N volts present to trip. Also they only trip on faults from live to circuit CPC, they offer no protection for live to true Earth.

So I would say they are a safety hazard in that case because they might lead someone to think they are protected but are not.
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Of course if the downstream board is RCD protected and it is only used as an isolator switch and labelled as such, then it would be OK.

 

[Mark Wright]

If there is no (usable) earth connected they will not work! That is a fundamental difference compared to the modern style of RCD that generally only needs L-N volts present to trip.

So I would say they are a safety hazard in that case because they might lead someone to think they are protected but are not.

Yeah I mean you see them used for Isolation switches now not for Earth fault.

but I ask is there any harm having them connected for earth fault(earth connected) along side RCD? as means of extra protection? as they seem quite solid.

 

[Mike Johnson]

America has a big thing about arc-flash, there are a lot of injuries and deaths (comparatively speaking) related to that compared to here. I don't know why, maybe more high power machinery, maybe differences in electrical system design, etc.

It's because whenever a car has an accident in America it bursts into flames after it has hit the power pylon which collapses and falls onto a petrol station and everything explodes, you have not been paying attention.

 

[pc1966]

It's because whenever a car has an accident in America it bursts into flames after it has hit the power pylon which collapses and falls onto a petrol station and everything explodes, you have not been paying attention.

Only for the Pinto!

Ford Pinto Fuel Tanks: Epic Auto Failures

Ford neglected to add reinforcements to protect the easily ruptured fuel tank, endangering drivers while earning the Pinto a reputation for catching fire that persists today. The automaker's public relations black eye lasted for years.

www.popularmechanics.com

(In fact the death rate was not so dramatically high but the PR impact was immense)
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Yeah I mean you see them used for Isolation switches now not for Earth fault.

As an isolator fine, saved troubling the DNO when doing a CU change.

but I ask is there any harm having them connected for earth fault(earth connected) along side RCD? as means of extra protection? as they seem quite solid.

Depends. If the property has been moved to TN-S/TN-C-S you would be introducing an intolerable earth impedance by it and so depending on its action for any non-RCD circuits. As they are quite old and often not routinely tested I would not be happy with that.

If it is still on a TT system you have the same underlying issue, but now you would expect a new up-front RCD and/or all downstream to be 30mA protected, so the loss of low-Z earth is not as serious.

However, it would be hard to argue this was making things safer. Say you have a 100mA S-type RCD incomer for the house and no other RCD on something like a fixed heater. If the VOELCB coil goes open you are then depending on the 100mA to clear any fault on that circuit and without the CPC linked to ground that means any metalwork stays live unless some other path to Earth is low enough. And that is a shock hazard due to the trip threshold of the main RCD.

 

[Mark Wright]

Ok thanks, there probably more trouble than there worth then - I did think the principle seemed really good actually checking the Voltage of the Earth.
Thanks.

 

[pc1966]

The idea of monitoring the actual earth conductor current has its places, for example the likes of the Zappi charger that has various PME fault protection steps as there it is the CPC itself presenting a shock risk if it becomes elevated in voltage.

But like many other issues of safely measuring potentially high currents in power circuits it is best to use a current transformer arrangement so the through-path is unlikely to fail or be damaged by high fault currents, then use some electronics to trigger on the smaller CT output.

 

[derek]

America has a big thing about arc-flash, there are a lot of injuries and deaths (comparatively speaking) related to that compared to here. I don't know why, maybe more high power machinery, maybe differences in electrical system design, etc.

It has been alleged that the USA has a very casual approach to the importance of thoroughly testing electrical installations and that if the circuit breaker holds all is good, meaning that AFDDs are far more important there to prevent fires caused by poor terminations. That's not to say they wouldn't help in this country as well especially if they can disconnect the supply to appliances that are dangerous.

 

[Mike Johnson]

I don't know the requirements for testing in the USA, but have seen advertisements on e-bay for second hand MFT's in the USA so assume they do have a regime for testing?

 

[marcuswareham]

It has been alleged that the USA has a very casual approach to the importance of thoroughly testing electrical installations and that if the circuit breaker holds all is good, meaning that AFDDs are far more important there to prevent fires caused by poor terminations. That's not to say they wouldn't help in this country as well especially if they can disconnect the supply to appliances that are dangerous.

Have you seen JWs video on the AFDDs, it would appear as if a few of them will allow enough arching to cause a fire before they trip :- S
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Ok thanks, there probably more trouble than there worth then - I did think the principle seemed really good actually checking the Voltage of the Earth.
Thanks.

JW also does a very good video on pointing out the flaws behind the VOELCBs

View: https://www.youtube.com/watch?v=JMh684HniF0

 

[pc1966]

I also remember reading this on how the USA breakers (of the time, it is 1990 here, might have improved now) let through a lot more fault energy that our MCBs:

https://paceforensic.com/pdfs/Circuit_Breakers_The_Myth_of_Safety.pdf