Discuss Up front Rcd question in the UK Electrical Forum area at ElectriciansForums.net
Correct ! Sorry my sloppy wordingAn RCD cannot control touch voltages - all it can do is disconnect.
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.
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.
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.
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.
100mA time delay main switch and a dual rcd db is all you need for this tt.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.
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 appearAn RCD cannot control touch voltages - all it can do is disconnect.
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 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.
Okay, but you're still going to get enough of a belt to kill you...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...
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 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 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.
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?
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?
Reply to Up front Rcd question in the UK Electrical Forum area at ElectriciansForums.net
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