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

 

[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.