Steel Bath how to earth.

[sparkticus]

We know from the touch-voltage equation that if your 'Earth Loop Impedance' (the 'earthing' bit), is less than 1667 ohms, then a 30mA RCD will disconnect before the voltage reaches 50 Volts, (what they deem as safe).

I think the RCD will initiate disconnection when the touch voltage is at or approaches 50 volts based on 50/1667 = 30mA. But the actual touch voltage will probably be higher than 50 volts for a short period. The RCD can not limit voltage or current - it can only attempt to disconnect as quickly as possible when the threshold is met.

 

[sparkticus]

We know from the touch-voltage equation that if your 'Earth Loop Impedance' (the 'earthing' bit), is less than 1667 ohms, then a 30mA RCD will disconnect before the voltage reaches 50 Volts, (what they deem as safe).

I think the RCD will initiate disconnection when the touch voltage is at or approaches 50 volts based on 50/1667 = 30mA. But the actual touch voltage will probably be higher than 50 volts for a short period. The RCD can not limit voltage or current - it can only attempt to disconnect as quickly as possible when the threshold is met.

In fact (having thought about it) an RCD would need to disconnect (after the threshold is met) within much less then 5 milliseconds to prevent a voltage of full amplitude (related to 90 deg phase angle) from passing through and even that assumes a phase angle of zero or 180 at the instant of the disconnection.

 

[Electrifying]

I think the RCD will initiate disconnection when the touch voltage is at or approaches 50 volts based on 50/1667 = 30mA. But the actual touch voltage will probably be higher than 50 volts for a short period. The RCD can not limit voltage or current - it can only attempt to disconnect as quickly as possible when the threshold is met.

Yes, we know that 'earthing' and the 'protective device' limit the 'duration' of the fault, where 'bonding' limits the 'magnitude' - but by limiting the 'duration', the RCD indirectly limits the level that the voltage reaches.

That's why it has to comply with 0.03 x 1667 <= 50v - to keep the voltage below 50 volts.

Otherwise, you'ld be able to have a Zs of 7667 ohms......... 0.03 x 7667 = 230 volts.

With only 30mA having to flow to trip the RCD within this time, no supplementary bonding is required.

A lot more current has to flow for a lot longer to trip the MCB in 0.4 sec, giving rise to dangerous voltage, so supplementary bonding required in a bathroom.

 

[Electrifying]

In fact (having thought about it) an RCD would need to disconnect (after the threshold is met) within much less then 5 milliseconds to prevent a voltage of full amplitude (related to 90 deg phase angle) from passing through and even that assumes a phase angle of zero or 180 at the instant of the disconnection.

You're going to the ridiculous, now, if you're going to start talking about 'actual' voltage, as opposed to nominal voltage to earth.

All calculations, tables etc etc are based on nominal, so lets stick with that.

I'm just trying to explain to John, simply, why the three requirements need to be met.

 

[sparkticus]

That's why it has to comply with 0.03 x 1667 <= 50v - to keep the voltage below 50 volts.

OK but in that case I don't understand. When an RCD "sees" something approaching 30mA it will start to disconnect. It may take 0.04 seconds to actually disconnect. That still allows time for 2 cycles of AC to pass. That is not 50 volts (RMS) that is approximately 230 volts (RMS)

Bonding (as I understand it) only reduces the intensity of a possible shock because it attempts to keep metal parts as a similar potential. The actuall potential on a metal case of an electric kettle (with a phase-earth fault) can still rise to approximately 230 volts (RMS) for up yo 2 cycles of AC within the disconnection time of 0.04 seconds. e

Yes the RCD will start to disconnect at or below 50 volts but the generator at the power station is going to keep running and up to two cycles will pass.

 

[sparkticus]

You're going to the ridiculous, now, if you're going to start talking about 'actual' voltage, as opposed to nominal voltage to earth.

I am trying to be accurate. there are many that do not understand the nature of an RCD. there are electricians out there that think that an RCD limits the fault current to 30mA. They (innocently) tell people that an RCD will protect people from receiving an electric shock. We need to talk beyond the regs.

 

[sparkticus]

All calculations, tables etc etc are based on nominal, so lets stick with that.

I am fully aware of that. But as I have been saying, there needs to be scientific discussion beyond what the red (soon to be green) book says.

 

[Electrifying]

I'll concur - because part of my 'facts' were wrong anyway.

I can't believe you didn't pull me on the obvious -

With only 30mA having to flow to trip the RCD within this time, no supplementary bonding is required.

So, we'll correct that to 150 mA has to flow to trip the RCD within 0.04 Sec.

But, still a lot less than for an MCB, and a lot faster - so my statements still stand, although I do see your point.

 

[sparkticus]

All calculations, tables etc etc are based on nominal, so lets stick with that.

I'm just trying to explain to John, simply, why the three requirements need to be met.

I'm just trying to explain to John, simply, why the three requirements need to be met.

Understood but I think you will find (I don't want to speak for him) that he fully understands the fundamentals. I believe he was asking a much more in-depth question which I then picked up on. In fact I completely agree with him and I support questioning the underlying science the underpins the requirements. Not for the sake of antagonising but to learn. From that perspective I do not want to stick with just the calculations/tables based on nominal voltages. I could just sit and read the regs all day if that were the case. I enjoy discussions that explore the topics in some depth with a bit of spirit as I am sure you do to.

 

[sparkticus]

I can't believe you didn't pull me on the obvious -

I was being polite

 

[matt1e]

absolutely ... and this is also only on condition that Main bonding fully complies with 411.3.1.2

Indeed - but, again, the same question - can you make any sense of the regulation in question?

Kind Regards, John.

yes its a bit like "yes you can do this but only if you have a backup plan"

Matt

 

[JohnW2]

You're thinking too much into it - it's simpler than you think.

I wouldn't say I was thinking too much - I was responding specifically to your suggestion that more rapid disconnection due to an RCD would lead to a lower p.d. - which doesn't male sense to me. To remind you, you had written:

Maybe you're missing the fact that without RCD protection the disconnection time under earth fault conditions will be a lot higher (i.e. within 0.4 sec as opposed to 0.04 sec), so the rise in difference in potential between an 'exposed conductive part' (the 'fault') and other metal parts could be a lot higher due to the prolonged fault condition.

That, to which I responding, is is clearly totally different from the argument you are now presenting.

We know from the touch-voltage equation that if your 'Earth Loop Impedance' (the 'earthing' bit), is less than 1667 ohms, then a 30mA RCD will disconnect before the voltage reaches 50 Volts, (what they deem as safe). .... Without an RCD, you cannot guarantee disconnection before a dangerous potential difference is reached .....

That is surely the classic misconception about RCDs? .... it would only be true that the RCD will disconnect "'before the voltage reaches 50v" if the touch voltage increased slowly, which is not what usually happens. When, as is the case in practice, faults usually appear suddenly and completely, all one can say is that, with a EFLI <1667&#937;, the 30mA RCD will disconnect if the touch voltage (at the point where the EFLI was measured) is >50V. If, as is the common situation with faults, there is sudden low impedance (regs talk about zero impedance) L-E fault, then the touch voltage will surely rise the the full supply voltage (230v or whatever, within a maximum of 0.005 sec {quarter of a cycle}) until the RCD operates.

... so you have to supplementary bond, in order to 'additionally' reduce this 'difference' - that's why, like a 30mA RCD, supplementary bonding is called 'additional protection'
It's applied to a bathroom because of the obvious additional danger.

Another strange thing about 701.415.2(v) (the requirement for RCD protection, amongst other things, if one is to avoid the need for supplementary bonding) is that 701.413.3.3 already (and very reasonably) requires RCD protection in bathrooms, under any circumstances - such that 701.415.2(v) is theoretically redundant in a compliant installation.

Kind Regards, John.

 

[JohnW2]

Duplicate deleted

 

[JohnW2]

... I'm not sure why its absence should cause one to have to locally bond e-p-cs which are already (as required by 701.415.2(vi) effectively bonded together. Perhaps I'm missing something.

I think I've probably more-or-less worked out the answer to my own question ....

... in essence, 'what I was missing' was the impedance of the CPC. Even if all the extraneous-conductive parts are effectively connected together (and connected to MPB/MET), then the p.d. which can arise between those e-p-cs and an exposed-conductive-part which becomes live is primarily determined by the impedance of the CPC which is earthing that exposed-conductive part. If the EFLI measured at the exposed-conductive part is low, and hence fault current large, then this p.d. (essentially the voltage drop along the CPC) can be very high. For example, with just 10 metres of 1mm² CPC, at a fault current of 1000A, the voltage drop, hence p.d. of interest in the bathroom, would by my reckoning be around 220v. With fatter CPCs and/or shorter CPC and/or lower fault current, the p.d. would be lower than this, but often still very substantial.

I suppose the 'worst' scenarios occur at 'medium' levels of EFLI - low enough to result in fairly high fault currents (hence fairly high CPC voltage drop, hence 'touch voltages'), but high enough to result in fault currents which are not high enough to result in very rapid operation of an MCB.

That being the case, it is perhaps reasonable that there is a requirement for the rapid disconnection provided by RCD protection if one does not have supplemenatry bonding. As I've noted elsewhere, this requirement in 701.415.2 is theoretically a bit redundant, since 701.411.3.3 already requires RCD protection in bathrooms under any circumstances, but I suppose such repetition does no harm.

Kind Regards, John.

 

[JohnW2]

I'm sure that I won’t always be here

Oh you will
I joined to pass a bit of time while I was off work for 5 weeks with a broken ankle. That was heading on for 6 years and 13 thousand posts ago
I love it here how much it makes you actually think about why you are doing things and personally I think being a contributer to this forum has made me a better electrician, and I certainly have a better understanding of the science of electricity, it's distribution and BS7671 too.

I'm not so sure (about me). I'd certainly like to remain here (since I 'enjoy' it in many ways) but, as I said, the subject in question is not actually a significant part of my life, and I have to give considerwtion to the rest of my life. For you, it makes total sense, for the reasons you give - but the difference is that 'matters electrical' are a significant part of your life!

I'm only teasing by the way. I'm sure you know you can rattle on a bit but your posts make for interesting reading.

Some things don't really need saying. Thanks

Kind Regards, John.