Up front Rcd question

[Dirk boddington]

After talking to someone who's Rcd failed them recently during a regular test it got me thinking.

I have a TT system and regularly test my rcd however if it failed to operate during a phase to earth fault then there is no backup.

I considered upgrading the consumer unit and adding rcbos but that's well beyond my budget right now. (they may fit in mine now but it will be very tight)

My second thought was adding a rcd in line with my existing consumer unit to act as a backup. All of my circuits are fed from this single rcd at present as its not a split load unit.

I've spoke to several electricians about it and I've had varying answers so thought I would ask a wider audience.

Can this be done with 2 30ma rcd or would this create nuisance tripping or would one have to be a 100ma?

 

[Risteard]

An RCD cannot control touch voltages - all it can do is disconnect.

 

[Wilko]

An RCD cannot control touch voltages - all it can do is disconnect.

Correct ! Sorry my sloppy wording

 

[D Skelton]

I take the points mentioned.

I think we are all agreed, that 100mA RCD will not protect a person from potentially receiving a fatal electric shock, whereas a 30mA RCD will provide that protection.

Yes we are

In a possible scenario (without human interaction) of a 30 mA RCD failing to function, at the time of a fault , the 100mA will eventually trip. Jobs a good'un.

However, in the scenario with human interaction, and the 30mA failing to function, the human will receive a possible fatal electric shock, before the 100mA functions.

As an S Type RCD, has not been designed for the above scenario, i.e. designed for personnel protection, so therefore why should it be conceived as fit for purpose, to provide 'back-up'. What if the 100mA fails at the same time, what's the 'back-up to the back-up'? To provide 'back-up', or fail safe suggests that the secondary device will provide protection if the primary device fails.

It will provide protection, albeit not ideal protection, but protection nonetheless. In your scenario with human interaction, if no s-type is fitted and the 30mA RCD fails, then there will be no disconnection. At least an s-type will provide disconnection within 500ms, which might not be perfect, but it's far better than the alternative.

Further, the idea behind providing back up protection is that in the event of a fault occurring and the 30mA RCD failing, the s-type will clear the fault in most cases before there is human interaction.

As said, the OP can do what he wishes, but it should not be suggested to design a system, which has no endorsements from manufacturers or recommendations from BS7671.

So your advice is basically; "because BS 7671 doesn't make any recommendations about providing some sort of back up protection in TT systems, don't make the installation safer"?

Sound advice that is!?

The point I'm making is simply this; RCDs are extremely prone to failure, so in an installation where you are relying on a single RCD as your SOLE means of fault protection, it would be prudent to allow for some form of fail safe mechanism. A cost effective means of achieving this is an s-type RCD up front.

The s-type is merely there as a last resort, nothing more. But it's a last resort that could easily save someone's life should the 30mA RCD fail as they often do.

 

[Midwest]

Yes we are



It will provide protection, albeit not ideal protection, but protection nonetheless. In your scenario with human interaction, if no s-type is fitted and the 30mA RCD fails, then there will be no disconnection. At least an s-type will provide disconnection within 500ms, which might not be perfect, but it's far better than the alternative.

Further, the idea behind providing back up protection is that in the event of a fault occurring and the 30mA RCD failing, the s-type will clear the fault in most cases before there is human interaction.



So your advice is basically; "because BS 7671 doesn't make any recommendations about providing some sort of back up protection in TT systems, don't make the installation safer"?

Sound advice that is!?

The point I'm making is simply this; RCDs are extremely prone to failure, so in an installation where you are relying on a single RCD as your SOLE means of fault protection, it would be prudent to allow for some form of fail safe mechanism. A cost effective means of achieving this is an s-type RCD up front.

The s-type is merely there as a last resort, nothing more. But it's a last resort that could easily save someone's life should the 30mA RCD fail as they often do.

I acknowledge your idea, that this design would cause the isolation of the install in the event of a fault and a failure of the 30mA RCD. I also acknowledge that RCD's, like any similar device is prone to failure. This demonstrates the importance of regular testing of them, something advised by the manufacturers.

I respect your views and knowledge, which you seem not to do of others. You often do make well informed remarks as an electrician, but suggesting the use of an RCD, not intended for personnel protection, would mitigate the effects of an electrical shock to a human, is even outside your expertise. All the articles I've read on the risk of electrocution, talk of a very small continuous electric current, 40mA and above, at just over 40ms;
' to cause irreversible damage to the normal cardiac cycle (‘ventricular fibrillation’) or death (‘electrocution’).

I'm no physician, so I'll tend to stick with that advice, and not suggest anything else.

I'm not saying it's a bad design, but it might not prevent fatal consequences in the scenarios discussed. Something the OP should be clear of, and not muddled by an incorrect assessment of the risks of electrocution.

Regular testing of RCD's is important in all designs, and all types of supplies.

 

[hightower]

So what happens if an RCD fails in a TN system? Okay the breaker might disconnect but if the fault is caused by human touch you're still gonna get a couple hundred amps up your arm. I'm having a hard time grasping people's arguments. All you can do is regular testing of the RCD and ensure your bonding is up to scratch. I agree that a 100mA is better protection than none but also agree that it could still cause someone's death. So what are we to do? Start installing RCDs in series as back up? Start installing RCD monitoring devices? Or do we do as suggested to us and push for correct bonding and regular testing. The amount of installs I've seen with no bonding, I'd argue that would cause a death long before a failed RCD would.

 

[Ian1981]

So what happens if an RCD fails in a TN system? Okay the breaker might disconnect but if the fault is caused by human touch you're still gonna get a couple hundred amps up your arm. I'm having a hard time grasping people's arguments. All you can do is regular testing of the RCD and ensure your bonding is up to scratch. I agree that a 100mA is better protection than none but also agree that it could still cause someone's death. So what are we to do? Start installing RCDs in series as back up? Start installing RCD monitoring devices? Or do we do as suggested to us and push for correct bonding and regular testing. The amount of installs I've seen with no bonding, I'd argue that would cause a death long before a failed RCD would.

100mA time delay main switch and a dual rcd db is all you need for this tt.
(Protective bonding as Hightower says as well)
Op doesn't want rcbo s for individual circuits so it's a no brainier for me.
Too be fair there's no requirement to do anything one rcd not ideal but not a massive issue

 

[Ian1981]

An RCD cannot control touch voltages - all it can do is disconnect.

True but the 50v is there to satisfy the calculation of 50/Idelta N giving you the max resistance before the rcd will disconnect in the required time before dangerous voltages appear
Or in this case Ra xIdelta N =<50 volts

 

[DPG]

So what happens if an RCD fails in a TN system? Okay the breaker might disconnect but if the fault is caused by human touch you're still gonna get a couple hundred amps up your arm. I'm having a hard time grasping people's arguments. All you can do is regular testing of the RCD and ensure your bonding is up to scratch. I agree that a 100mA is better protection than none but also agree that it could still cause someone's death. So what are we to do? Start installing RCDs in series as back up? Start installing RCD monitoring devices? Or do we do as suggested to us and push for correct bonding and regular testing. The amount of installs I've seen with no bonding, I'd argue that would cause a death long before a failed RCD would.

No you're not - Ohm's law will prevent it

 

[D Skelton]

I acknowledge your idea, that this design would cause the isolation of the install in the event of a fault and a failure of the 30mA RCD. I also acknowledge that RCD's, like any similar device is prone to failure. This demonstrates the importance of regular testing of them, something advised by the manufacturers.

I respect your views and knowledge, which you seem not to do of others.

I respect the views and knowledge of anyone who's views aren't formed on the basis of a misinterpretation of mine, and who's knowledge is formed on the basis of objective truth.

You often do make well informed remarks as an electrician, but suggesting the use of an RCD, not intended for personnel protection, would mitigate the effects of an electrical shock to a human, is even outside your expertise.

No it's not. It's entirely correct to say that the use of a time delayed RCD will mitigate the effects of an electrical shock to a human far better than nothing at all.

All the articles I've read on the risk of electrocution, talk of a very small continuous electric current, 40mA and above, at just over 40ms;
' to cause irreversible damage to the normal cardiac cycle (‘ventricular fibrillation’) or death (‘electrocution’).

I'm no physician, so I'll tend to stick with that advice, and not suggest anything else.

This information is most likely correct, and is in line with all the information I have read on the effects of electric shock. That said, it's entirely irrelevant to the debate.

I'm not saying it's a bad design, but it might not prevent fatal consequences in the scenarios discussed. Something the OP should be clear of, and not muddled by an incorrect assessment of the risks of electrocution.

No incorrect assessment on the effects of electric shock has been made. The point which you continually seem to miss is that a time delayed RCD will do a far better job of preventing electrocution upon the failure of a 30mA RCD than nothing at all.

Installing an up front time delayed RCD to a TT installation reliant on a single 30mA RCD for fault protection is not a substitution for anything, and is certainly no substitution for regular testing of the RCDs, it's an addition. An addition which will increase the safety of the installation exponentially.

Regular testing of RCD's is important in all designs, and all types of supplies.

I agree

 

[DPG]

Got to say I agree with Mr. Skelton - I would sooner rely on a time-delayed RCD than nothing at all.

 

[hightower]

No you're not - Ohm's law will prevent it

Okay, but you're still going to get enough of a belt to kill you...

 

[DPG]

Okay, but you're still going to get enough of a belt to kill you...

Yes, indeed. Wasn't meaning to be picky, but it's amazing how many people think that you get a bigger belt from a cooker circuit rather than a lighting circuit, just based on the MCB rating.

 

[D Skelton]

So what happens if an RCD fails in a TN system? Okay the breaker might disconnect but if the fault is caused by human touch you're still gonna get a couple hundred amps up your arm. I'm having a hard time grasping people's arguments. All you can do is regular testing of the RCD and ensure your bonding is up to scratch. I agree that a 100mA is better protection than none but also agree that it could still cause someone's death. So what are we to do? Start installing RCDs in series as back up? Start installing RCD monitoring devices? Or do we do as suggested to us and push for correct bonding and regular testing. The amount of installs I've seen with no bonding, I'd argue that would cause a death long before a failed RCD would.

I think you're confusing additional protection with fault protection here buddy.

In a TN system, fault protection is achieved with the use of circuit breakers and/or fuses, and rarely with RCDs. In a TT system (99.9% of domestic TT systems at least), fault protection is more often than not provided by a sole 30mA RCD.

In the event of a L-E fault on a TN system the circuit breaker would disconnect, if that failed you potentially have an RCD, and after that you have the main fuse. The reason for this is because of the low earth fault impedance. A TT system rarely takes advantage of a low earth fault impedance, thus we mainly rely on RCDs, which are prone to faliure in ways circuit breakers and fuses are not.

I hope that clarifies things for you.

 

[Midwest]

I respect the views and knowledge of anyone who's views aren't formed on the basis of a misinterpretation of mine, and who's knowledge is formed on the basis of objective truth.



No it's not. It's entirely correct to say that the use of a time delayed RCD will mitigate the effects of an electrical shock to a human far better than nothing at all.



This information is most likely correct, and is in line with all the information I have read on the effects of electric shock. That said, it's entirely irrelevant to the debate.



No incorrect assessment on the effects of electric shock has been made. The point which you continually seem to miss is that a time delayed RCD will do a far better job of preventing electrocution upon the failure of a 30mA RCD than nothing at all.

Installing an up front time delayed RCD to a TT installation reliant on a single 30mA RCD for fault protection is not a substitution for anything, and is certainly no substitution for regular testing of the RCDs, it's an addition. An addition which will increase the safety of the installation exponentially.



I agree

Can't seem to do the multi quote thing. But lets keep it simple.

Everything I've ever read & heard on the subject of the use of RCD's to try and negate the risk of electrocution, state the use of 30mA RCD. How will the use of a 100mA RCD achieve this, if 40mA is enough to prove fatal?

 

[D Skelton]

Can't seem to do the multi quote thing. But lets keep it simple.

Everything I've ever read & heard on the subject of the use of RCD to try and negate the risk of electrocution state the use of 30mA RCD. How will the use of a 100mA RCD achieve this, if 40mA is enough to prove fatal?

You are also confusing fault protection and additional protection.

If it makes it easier, don't think of it as a 100mA RCD, think of it as a 0.1A circuit breaker that just doesn't provide overcurrent protection.

An RCD in a TT system (in the context of this debate) is there to provide fault protection, not additional protection. It just so happens that boards come preloaded with 30mA RCDs which in the case of a TT system kill two birds with one stone.

Assume a TT system has no requirements for additional protection. No sockets for general use, no buried cable, no bathroom etc... there's no requirement to use a 30mA RCD for fault protection. You could use a 100mA one, a 300mA one, a 1000mA one, assuming a suitable stable Ra value (500 ohms, 167 ohms, 50 ohms respectively)

 

[Midwest]

You are also confusing fault protection and additional protection.

If it makes it easier, don't think of it as a 100mA RCD, think of it as a 0.1A circuit breaker that just doesn't provide overcurrent protection.

My lack of knowledge on that subject to one side, if I touch a live wire with my hand and my bare foot touches earth, I'm led to believe the current flow would be insufficient to operate the fuse or ocpd, but would operate an RCD due to the imbalance. Or is that incorrect?

 

[DPG]

My lack of knowledge on that subject to one side, if I touch a live wire with my hand and my bare foot touches earth, I'm led to believe the current flow would be insufficient to operate the fuse or ocpd, but would operate an RCD due to the imbalance. Or is that incorrect?

I would say that is correct.

 

[D Skelton]

My lack of knowledge on that subject to one side, if I touch a live wire with my hand and my bare foot touches earth, I'm led to believe the current flow would be insufficient to operate the fuse or ocpd, but would operate an RCD due to the imbalance. Or is that incorrect?

I've edited and added some more to my previous response.

In answer to your question, you are correct, and in that scenario, the use of an RCD to prevent a shock as you describe is to use it for the purpose of additional protection.

Additional protection is not a factor in this debate.

 

[Midwest]

So therefore a 30mA RCD would limit the minimise the current flow to 30mA (or there about), whereas a 100mA would minimise it to 100mA (or there about).

40mA is sufficient to prove fatal.

 

[Ian1981]

30mA rcds need a flow and return of the same current in line and neutral if you touch a live part then some current will flow through you to earth and not return through the rcd.
Rcd will detect an imbalance between live and neutral and operate the coil,as long as it 30mA or near that and turn off the device.
Think that's the basic way of explaining it anyway.
Apologies if you know already know this

 

[D Skelton]

Okay this is becoming tiresome. Midwest, with the greatest of respect, you need to learn the difference between additional protection and fault protection.

Once you do, I assure you your lightbulb moment will come.

 

[Midwest]

There is no need to be insulting. I'm just trying to understand how suggesting a 100mA RCD would provide protection against electric shock in the scenarios given?

 

[Ian1981]

There is no need to be insulting. I'm just trying to understand how suggesting a 100mA RCD would provide protection against electric shock in the scenarios given?

It won't is the short answer it's for fault protection only.
Or in the terms of a time delay discrimination as well

 

[D Skelton]

On the contrary, it is not my intention to insult you. I simply realised that until you understand the fundamental concept of fault protection, what I'm saying will be going over your head.

I have made no suggestion that a 100mA RCD will provide additional protection (BS 7671 definition) against electric shock, only that it will provide a form of back up fault protection.

 

[Midwest]

The s-type is merely there as a last resort, nothing more. But it's a last resort that could easily save someone's life should the 30mA RCD fail as they often do.

I'm sorry, I seem to have misunderstood your suggestion.

 

[D Skelton]

You most certainly have.

May I suggest you read up on fault protection as once you do, what I'm saying will become a lot clearer.

(that's a genuine suggestion and not meant to be read in a patronising tone btw)

 

[Midwest]

Before I go off and read up on fault protection;

So you weren't suggesting that using 100mA RCD would provide some measure of additional protection, in the event of the 30mA RCD failure?

 

[D Skelton]

No, I was suggesting that a 100mA s-type would provide a form of back up fault protection.

I stated that it would be an additional life saving safety measure rather than a substitution for one, but not that it would provide 'additional protection' as per the BS 7671 definition of 'additional protection'.

 

[Midwest]

Oh well that's clarified that then. Still not quite sure about the 'additional life saving safety measure', but I supposed we've done that to death, if you pardon the pun.

 

[D Skelton]

Still not quite sure about the 'additional life saving safety measure'

By providing a back up form of fault protection should the sole means of fault protection which is prone to failure fail.

This is an additional safety measure that should the sole means of fault protection fail, would most likely be life saving.