PE advice by Napit (hot tubs)

lyledunn: 
 

Why am I not seeing your links?!

On my iPad there are no links other than the one by James Follon. The link below your last post JP is also not visible on mi iPad. All links are visible on my PC. Pity because I do most of my forum browsing on my Ipad. 

Seems like the full link address as James posted is needed for the Ipad. 

These things used to work, but no longer do so on my iPad. Interesting article in Wiring Matters!

#OffTopic viewing links

Hi all just to let you know that @Elizabeth Morgan and myself are on it. We've taken a look and we think we've found a quirk in that shared links are not appearing on Apple devices (but bizarrely do for any other make of phone or tablet!) 

I've managed to work around it and exposed the links now by removing the link in the orginal posts and then reinserting them by using the link icon and then pasting as plain text into the field. Pasting as rich text will cause the link to reformat using the open graph protocol which is what I think is causing the issue with apple devices. 

I'm unsure if this is an issue with our platform or if it's something related to Apple's latest iOS update which I know has happened over the last few days (as I've updated all of my own apple devices just recently). So if there's anyone that hasn't updated to the latest iOS version (14.7.1)  but IS seeing the issue then please let us know! 

We'll be contacting our supplier to see what they say.

Thanks muchley! ?

John Peckham: 
 

Evening All

You may find this article in IET Wiring Matters more useful.  

https://electrical.theiet.org/wiring-matters/years/2021/86-july-2021/hot-tubs/

It does cover PME earthing.

If you register with the IET to receive Wiring Matters it will come to you via an email.

JP

If we're worried about perceived electric shocks - where it sounds like having an earthing system even a few volts above true Earth could be a problem - then is a TT system really any better than PME? On a TT system earth leakage currents (either from filters etc or just damp getting into things) will raise the voltage on the Earthing system (Ohm's Law - leakage current x Ra) - say 50mA and 200 Ohms would produce 10V - and for indefinite lengths of time.  BS 7671 only require we keep it below 50V.

   - Andy.

gkenyon: 
 

aligarjon: 
 

Pretty sure NIC did a webinar a while back about hot tubs and said it was ok to use the PME earth, i think because there was no exposed metalwork on them but can't remember . Have to say i wouldn't personally.

 

Gary

Is that true for ALL hot tubs? I saw a TV series this week about the subject, and there was a stainless-steel hot tub … also, there are exposed-conductive-parts on some heaters used in the devices ?

I think the real situation is far more tricky to navigate.

Probably, but that's true with most of what we do, no two situations are the same and we have to make a judgement using our experience and knowledge.

Gary

AJJewsbury: 
 On a TT system earth leakage currents (either from filters etc or just damp getting into things) will raise the voltage on the Earthing system (Ohm's Law - leakage current x Ra) - say 50mA and 200 Ohms would produce 10V - and for indefinite lengths of time.  BS 7671 only require we keep it below 50V.

   - Andy.

Does BS 7671 really require we keep the touch voltage below 50 V? I accept that, in general, for “leakage” currents, it is likely to be the case.

But I'm sure BS 7671 doesn't even specifically require supplementary local equipotential bonding to achieve that under all conditions - although it's likely in a number of cases to be achieved, it's not guaranteed. The fault current causing operating of a device, Ia, may well be far below the prospective fault current, especially where RCDs are used … if OCPDs are used then it's the current causing the device to operate in 5 s …

Worth remembering that for ADS we do calculations using worst-case (lowest) prospective fault current (i.e. that to the measured earth electrode resistance), but when we connect extraneous-conductive-parts, particularly if the TT is derived from a PME system, and we share extraneous-conductive-parts with a PME installation, it's entirely possible the prospective fault current and loop impedances could effectively pretty much revert back to PME values … 

So, how's this relevant?

Well, using Reg 415.2.2 if I have a 30 mA RCD (in either a TT or TN system), I can “get away with” bonding resistances in excess of 1667 ohms - perhaps more than average dry body impedance !!!!!

Now, I'm not saying BS 7671 is in any way wrong, or that there's an issue here, but “50 V” isn't the requirement …

Does BS 7671 really require we keep the touch voltage below 50 V?

For TT systems that use RCDs, and for ‘persistent’ touch voltages , yes - 411.5.3 (ii).

The TT systems that have such low loop impedances that overcurrent devices can be used, there isn't a specified limit, but as the system then approximates to TN systems any reasonable leakage current isn't likely to do any harm.

    - Andy.

AJJewsbury: 
 

Does BS 7671 really require we keep the touch voltage below 50 V?

For TT systems that use RCDs, and for ‘persistent’ touch voltages , yes - 411.5.3 (ii).

The TT systems that have such low loop impedances that overcurrent devices can be used, there isn't a specified limit, but as the system then approximates to TN systems any reasonable leakage current isn't likely to do any harm.

    - Andy.

Nope … this is effectively the same requirement as 415.2.2 …

… where the fault current exceeds the residual operating current, the touch voltage may also exceed 50 V until the device trips.

But also, this is only the earth electrode resistance, not the effective resistance from the transformer earth to the point at which you are standing … 

Just to be clear regarding this, yes, you're unlikely to get a voltage greater than 50 V to “Earth" (well, the other end of the earth electrode resistance sphere of influence) … but I think that Supplementary Local Equipotential Bonding is far greater an issue.

Basically, your touch current can exceed Ra … for a number of reasons (as I explained in the previous post, this completely negates the earth electrode resistance itself from the loop) … and if you've selected a bonding resistance (permitted by BS 7671) in excess of 1K, you may have a touch current that would be OK in dry conditions, but not wet or submerged as we are discussing here.

Some of this is perhaps crossing out of BS 7671 into product standards for hot tubs themselves, but does need to be considered somewhere along the line.

where the fault current exceeds the residual operating current, the touch voltage may also exceed 50 V until the device trips.

But the touch voltage won't persist - as the RCD will trip. Remember I was comparing the situation with the problem of perceived shocks on PME systems - due to voltage drop along PEN conductor in normal, non-fault, conditions.

We say to avoid PME partly because perceived electric shocks from the PEN conductor which can potentially be up to (say) 18.4V above Earth, yet we recommend a TT system - possibly TT'd an entire installation - where there's no guarantee that voltages of anything up to to 50V might persist. 

  - Andy.