Another Hot tub install

[Mike Johnson]

Thanks for digging that out, to me it only shows the phase being coil controlled by the large coil on the back of the unit, the smaller wire coil and toroidal unit controlling the arc disconnection, by my estimation to be a truly dual pole breaker there would have to be two large coils controlling the phase and neutral independently, supposition on my part, but that's where I was coming from in my previous post when I posted that the unit needing to be two module in width, there have been lots of claims of dual RCBO's prior to this point, but as I posted above they need to separate the contacts by 3mm to conform to the EU standard, I certainly look forward to someone finding one, as I have run out of room in my CU.
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Difficult to see the circuit diagram on the side of the FuseBox RTAMB, but it does look as though it is double switched, by one coil, would be interested in the separation gap of the contacts, I will try to contact them tomorrow.

 

[nicebutdim]

I've attached the data sheet for that Fusebox RCBO, which shows its circuit diagram.

It's interesting that Crabtree, Wylex & Fusebox label these RCBOs as 1P+N, but they're often advertised by wholesalers (who should know better) as DP.

My initial interest in this type of breaker stemmed from trying to find something that would offer additional fault protection on a TT system and settled on Crabtree, due to Starbreaker's main switch being on the left side. I was happy that these switched both poles in an overcurrent situation, but hadn't thought much beyond that. Combined with a time delayed upstream RCD, they're certainly better than what was replaced.

 

[Mike Johnson]

I've attached the data sheet for that Fusebox RCBO, which shows its circuit diagram.

Well that one certainly only shows the phase being switched and is denoted as 1P+N will assume it's not the new compact RTAMB type except that is the only Fusebox RCBO data sheet I can find also.

Even the Crabtree Starbreaker is denoted as 1P+N on it's data sheet, it makes me wonder if the N is electronically disconnected and not a physical 3mm separation.

https://www.tlc-direct.co.uk/Technical/DataSheets/Crabtree/Crabtree_Compact_RCBO.pdf

 

[stevethesparks]

I think an electronic disconnection would surely not be allowed? Wylex state no need for disconnection when testing so to me indicates proper isolation. Regarding a 3mm gap I couldn’t say but again would assume they would have to comply?

 

[Mike Johnson]

Agreed, but on the circuit diagram/s they only show one physical disconnection.

I just looked at the Hager two module RCBO data sheet and that says 1P+N, but N switched concurrently, could only find it on an African site, will look for a EU data sheet to see if it's different: https://www.africa.hager.com/bs/files/download/0/78809_1/0/AD906J_Data_Sheet.pdf

Found an Irish site with the data sheet on, but that shows even less information: https://www.hager.ie/files/download/0/559125_1/0/AD906J_Data_Sheet.pdf

 

[pc1966]

but as I posted above they need to separate the contacts by 3mm to conform to the EU standard

At around 8:30 in that video you can see the neutral contacts is open by at least 3mm (more or less central to the picture). Only later in the video when they flip it over you see the thermal-magnetic trip side that opens the line and has the arc chamber.

Most likely it is based on the typical "switched neutral" style of switch where N makes first/breaks last compared to the line contact(s) so does not have to switch the majority of current (hence no arc chute).

 

[Mike Johnson]

Got hold of a Hagar AD906J an it shows a physical disconnection of both poles on the diagram on the front of the unit whereas most single module units show a pass through of the N.

 

[pc1966]

Got hold of a Hagar AD906J an it shows a physical disconnection of both poles on the diagram on the front of the unit.

It also shows the thermal/magnetic in only the left (line) path, so basically the same.

 

[Mike Johnson]

Sorry don't understand your post, same as what?

 

[pc1966]

Sorry don't understand your post, same as what?

Same as the Wylex/Crabtree/Fusebox units that are 1P + switched N.
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Though without opening the Hager one there is no indication if the N path really has an arc chute, etc, as for L.

 

[Mike Johnson]

Except the Wylex/Crabtree/Fusebox units do not show a physical disconnection of the N as they do for the line on their data sheets, so I am dubious that the N is switched, as I have said before, the physical size of the compact units may not be able to accommodate two switch units, it is supposition on my part as you say without taking a Hager unit apart it's difficult to say categorically, but I do think the diagram may be indicative of the units parameters.

All seems a bit strange to me, if these units are 2p switched why don't they just say so instead of 1P+N?
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Thinking about the previous question, I wonder if the difference is that the N does not have it's own fault circuit and is just dragged along with the line if a fault develops?

I think that is what N switched concurrently is supposed to mean, Hmmm

 

[pc1966]

Except the Wylex/Crabtree/Fusebox units do not show a physical disconnection of the N as they do for the line on their data sheets, so I am dubious that the N is switched,

But they do:

All seems a bit strange to me, if these units are 2p switched why don't they just say so instead of 1P+N?

Thinking about the previous question, I wonder if the difference is that the N does not have it's own fault circuit and is just dragged along with the line if a fault develops?

Exactly. N is switched, it is not tripping on an over-current fault alone. There is only 1 pole that is capable of sensing and fully interrupting the fault current.

In the diagram from Wylex you see the 'X' for the thermal trip is only on the L path ("LOAD 2" as they call it).
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Of course, as an RCBO if you do have a fault leading to N-E current it will trip.

But it would not be pretty if you had reversed polarity fault AND then a 'N' to E fault in the kA range!

 

[Mike Johnson]

So they do, thanks for that, it's been an interesting discussion, with I think a conclusion.

One must assume that the compact single module units are just a space saving exercise.

 

[HappyHippyDad]

that is a good idea providing both types of earthing but the written paperwork that has been provided to me by the customer from the hot tub company states that they do not provide any wiring and they require 5meters of flex left connected to the isolater. They just connect up the flex and if it’s not as requested then they charge for a return visit. So I get the feeling they are not interested what they have just as long as it looks like they normally have.

with regards to the DP RCBO I am going to go down that route but the customer is wanting another DB installing so they can fit a hob in the future as there current one is full so I was going to install a second consumer unit and use RCBOs.
I’m just not sure how these DP RCBOs fit do they need a separate enclosure or could I fit a consumer unit and cut the bus bar back and fit the DP RCBO at the end with a link from the neutral bar ?

The isolator will be at least 2 meters away from the tub and the socket 3 meters away.

I am still thinking of going down the supplementary rod road just incase the loss of a PEN conductor

I have just started a thread which has some similarities to this thread with regards the earthing. See in particular post 3 Understanding the relationship between TNCS, TNS and TT - https://www.electriciansforums.net/threads/understanding-the-relationship-between-tncs-tns-and-tt.188980/

It would seem that if you are going to extend the PME AND add an earth rod for the Hot tub you could well be making it more dangerous in the event of PEN failure. Unless of course you did get the Ra down to around 2 ohm (which isn't going to happen).

I have to admit I find it a bit confusing but if you Just made the new CU a TT and did not extend the PME then the new earth rod would not raise to 230V in the event of a PEN failure which seems like a better option!

If you TT the new install and don't extend the PME you rely solely on the RCD for fault protection.

If you extend the PME you have ADS and additional protection with an RCD but in the event of PEN it could become dangerous.

I think I would go for option one, TT for the new install. I'd probably run a 3 core 10mm SWA (and isolate the armour and 3rd core) in case I changed my mind!

 

[Midwest]

I thought I understood this subject some time back, but this thread has got me confused again.

What is the point of using a DP or SP & Switched Neutral RCBO for this hot tub supply & PEN conductor faults, if it was TT'd? Other than manufactures (i.e. Schneider) or disconnecting neutral for fault finding etc.

Where is this hot tub being installed, on what surface is it being placed? I recall reading a thread about faulty Lazy Spa hot tub, with voltages on the water, but all of these hot tubs are insulated, no metal surfaces. Not as if its like a ev car. If the bathers not stepping on bare Earth, would it not be better to rely on the PME, and not to be too concerned about a lost PEN conductor?

 

[stevethesparks]

pme is acceptable as is a SP rcbo, but nothing wrong with a belt and braces approach. Lost PEN may be quite rare but it still happens.
Iv'e always adopted the idea of treating a hot tub as a swimming pool.

Drifting slightly off the subject, it is strange that if you have a shower in a bedroom that a socket outlet requires to be no less than 3 metres away. However, with a swimming pool where it is not physically possible to locate a socket outside of zone 1, then it will be allowed 1.25mts horizontally from zone 0???
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I missed part of your post. If a PEN fault occurs, then with a SP rcbo, this will still allow a path through the hot tub element/pump via the faulty neutral will it not?

 

[Midwest]

pme is acceptable as is a SP rcbo, but nothing wrong with a belt and braces approach. Lost PEN may be quite rare but it still happens.
Iv'e always adopted the idea of treating a hot tub as a swimming pool.

Drifting slightly off the subject, it is strange that if you have a shower in a bedroom that a socket outlet requires to be no less than 3 metres away. However, with a swimming pool where it is not physically possible to locate a socket outside of zone 1, then it will be allowed 1.25mts horizontally from zone 0???
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I missed part of your post. If a PEN fault occurs, then with a SP rcbo, this will still allow a path through the hot tub element/pump via the faulty neutral will it not?

I‘m not to familiar with hot tub designs, I would seek guidance from the manufacturer.

However, the point of lost pen conductor, and using a SP or DP device, would not make much difference surly, as both would still be energised during this situation. In this situation in is the still connected exposed conductive parts, extraneous conductive parts and true Earth, that’s cause for concern, not the broken neutral.

Section 702 does not preclude the use of pme for swimming pools.

Most hot tubs I’ve seen have no exposed or extraneous conductive parts. As said, I’m not sure of the internal components design,.

 

[stevethesparks]

However, the point of lost pen conductor, and using a SP or DP device, would not make much difference surly, as both would still be energised during this situation. In this situation in is the still connected exposed conductive parts, extraneous conductive parts and true Earth, that’s cause for concern, not the broken neutral.
.

I agree. there is no guarantee that an rcd/rcbo would trip in such a circumstance, but my ideology is just to be able to provide total isolation, hence TT to isolate from the pme and DP to ensure total isolation of the supply.


You are correct also about pme not being precluded with swimming pools, but again that's something I would not do.

 

[pc1966]

In the TN case from PME then a DP breaker is not really an advantage here - if your PEN goes live your hot tub earth, etc, is still live with L & N open anyway.

But in the case of going TT to avoid the PEN fault risk you really must be DP breaking otherwise a 'live N' style of fault arising from the PME fault would not be isolated by the breaker opening.
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Just to add the RCBO would not open on a PME fault alone, but if you had another fault leading to earth leakage current at the same time (e.g. rise in L-true E volts causing breakdown, etc) you would still expect it to be safely disconnected.

 

[Mike Johnson]

In real terms how often does a PME fault occur? I have never experienced or seen one.