Initail Problem with Sauna Heater opens bag of worms!

[adderzigzag]

Sorry to post such a long message but realise people need all the information so accurate conclusions can be reached. The domestic property has a supply feeding some out building via three core cable from the house protected by a B40, more on that later! The supply to the house is single phase TN-CS (PME), this may also be relevant later!


I was called to help out with a problem on a tripping breaker in the house feeding the outbuildings, at that time it was a B40 RCBO (30mA) for obvious reasons. It was established that within the outbuildings the client had fitted a Sauna, a little look at Section 703 mentions RCD protection should NOT be fitted to the heater supply unless otherwise stated by the manufacturer.

Clearly the heating elements are inherently leaky and an Insulation Resistance Test confirmed that.

An IR test on the cable proved satisfactory.


I recommended a split consumer unit to be fitted in the outbuilding, change the feed breaker within the house from RCBO to straight MCB. Both have the same overcurrent rating.

The split board was then configured to provide a non-RCD supply to the heater with all other circuits on the RCD side.


A Zs was measured at a 13A socket outlet in the outbuilding at 8 ohms, not good for a B32 breaker 1.15 ohms max tabulated, however RCD was upstream considering this high earth fault loop impendence, think max is 1667 ohms, with RCD.


With some further investigation the 3 core cable to the outbuilding did not have the CPC connected to earth at the MET within house DB nor was the SWA connected, at the very least one end should be bonded as this is an exposed conductive part.

When questioning the client he said the installer said under no circumstances should this core be connected to the MET, outbuilding end is also unearthed. There is an earth rod at the outbuilding and the structural steel work also bonded.


OK my question and I know there are issues using PME with various Special Locations, like caravan parks, farms, marinas, plus others.


Why should the cpc on the feed cable not be connected at the MET at the outbuildings DB? This would drop the Zs dramatically to the outbuildings and hence aid disconnection time issues. So should I connect it or not? What would be the negative consequence of connecting it? Is it a PME ruling?


Look forward to any suggestions as currently the disconnection time on the feeder cable is likely to be pretty rubbish even considering it would be a 5s disconnection!

Hopeful for some good suggestions

 

[ImpededLoop]

The cpc should be connected at one end at least. What type of cable is it?

Are there any extraneous parts in the outbuilding? It may have been TT'd to save bonding them to the MET.

If it is TT you'll need to sort out something for the non RCD protected circuits in the outbuilding.

 

[Darkwood]

What is the size of the SWA 3core.. ?

The armour should be earthed at supply end regardless of the load end been isolated off and a TT set up etsablished, what I don't understand is why the TNCS equipotential bonding zone wasn't extended to the outbuilding and a suitably sized earth exported out to bond the steelwork, it really makes me wonder why people shun a excellent earthing system for a lesser reliable one.

What do the manufacturers guidance say about the sauna?

 

[davesparks]

The cpc and armour must only be connected to the MET if a seperate earthing system (TT) has been installed at the outbuilding.
All bonding and cpcs of the outbuilding should be connected to the EMT of that building.
An installation only has one MET which is at the origin, a similar point in an outbuilding where bonds etc are connected is an EMT (earth marshalling terminal)

The outbuilding has obviously been connected as a seperate TT system, the original installer was correct that the submain cpc must not be connected at the outbuilding in this system.
By removing the RCD protection you have taken a compliant installation and made it non-compliant and potentially dangerous.
For the safety of the users of the installation you would be better to employ the services of an experienced electrician who knows what they are doing with this installation rather than working outside of your own knowledge.
If it worked before and this fault is recently developed then you have most likely got a fault, not a design issue.

 

[HappyHippyDad]

Gosh, quite a few questions to bring up there!


1. What size is the SWA?
2. One end of the SWA does need to be connected to earth.
3. The RCBO may be tripping because the current is too high, not due to an earth fault, probably not, but worth checking.
4. Section 733.411.3.3 says RCD protection 'need not be provided' (for the heater) it doesn't say 'should not'. It also says RCD 'should' be provided for all circuits of the sauna.
5. Does it trip immediately or could you get a clamp meter on it?
6. What were you IR results for the heating element?

 

[marconi]

The cpc and armour must only be connected to the MET if a seperate earthing system (TT) has been installed at the outbuilding.
All bonding and cpcs of the outbuilding should be connected to the EMT of that building.
An installation only has one MET which is at the origin, a similar point in an outbuilding where bonds etc are connected is an EMT (earth marshalling terminal)

The outbuilding has obviously been connected as a seperate TT system, the original installer was correct that the submain cpc must not be connected at the outbuilding in this system.
By removing the RCD protection you have taken a compliant installation and made it non-compliant and potentially dangerous.
For the safety of the users of the installation you would be better to employ the services of an experienced electrician who knows what they are doing with this installation rather than working outside of your own knowledge.
If it worked before and this fault is recently developed then you have most likely got a fault, not a design issue.

By Marconi It seems to me that in situations like this the original installer of the wiring for the outbuilding was remiss because he failed to leave behind suitable notices at either end of the SWA cable to make it absolutely clear, what the design is, why terminations have or have not been made, quoting suitable references from BS7671 and recorded of course on the certificate he left behind.

 

[Lucien Nunes]

Heating element leakage can increase with age and infrequent use. If the RCBO was tripping specifically due to heater leakage, then it might have been OK at first but no good now, without a genuine fault being present. If the heater leakage is too high to live with a 30mA RCD but the Ze is too high to live without one, it might be possible to use an RCD with higher tripping current for the heater circuit (to satisfy disconnection time) and a 30mA one for other circuits (to provide additional protection). But I'd look at improving the earthing first.

 

[davesparks]

By Marconi It seems to me that in situations like this the original installer of the wiring for the outbuilding was remiss because he failed to leave behind suitable notices at either end of the SWA cable to make it absolutely clear, what the design is, why terminations have or have not been made, quoting suitable references from BS7671 and recorded of course on the certificate he left behind.

How do you know that it is not recorded on the certificate and the OP just hasn't asked to see the certificate?

 

[marconi]

By Marconi In the original post adderzigzag writes: 'When questioning the client he said the installer said under no circumstances should this core be connected to the MET, outbuilding end is also unearthed. There is an earth rod at the outbuilding and the structural steel work also bonded.'

Dear Daversparks - The OP wrote that he was informed by the client not to do something with the wiring. He does not mention any notices. He is not offered nor does he seek sight of the necessary certification. The client can move home. The certificate can get lost. My substantive point is there is not in place a reliable way of preserving this important information for future electricians undertaking work. But to answer your post of course I don't know for certain. Perhaps adderzigzag can do that for us?

 

[davesparks]

By Marconi In the original post adderzigzag writes: 'When questioning the client he said the installer said under no circumstances should this core be connected to the MET, outbuilding end is also unearthed. There is an earth rod at the outbuilding and the structural steel work also bonded.'

Dear Daversparks - The OP wrote that he was informed by the client not to do something with the wiring. He does not mention any notices. He is not offered nor does he seek sight of the necessary certification. The client can move home. The certificate can get lost. My substantive point is there is not in place a reliable way of preserving this important information for future electricians undertaking work. But to answer your post of course I don't know for certain. Perhaps adderzigzag can do that for us?

For such a simple installation any competent electrician should be able to recognise what has been done and work accordingly.
It's not exactly an uncommon method of installation!

 

[marconi]

For such a simple installation any competent electrician should be able to recognise what has been done and work accordingly.
It's not exactly an uncommon method of installation!

By Marconi Dear davespark, In this instance the OP did not recognise the design otherwise he would not have begun the topic and asked the questions of more knowledgeable folk like yourself. Sensibly he recognised his unfamiliarity with the wiring scheme and sought to clarify the wiring before he did anything to it. Other electricians, assuming themselves to be competent - but actually not - might have made connections incorrectly and thus prejudiced the safety of the installation and its users. I contend that simple signage/notices at either end of the SWA would have brought it to the attention of all and thus removed the risk of unconsciously-incompetent folk doing something dangerous and or troublesome.

 

[davesparks]

By Marconi Sensibly he recognised his unfamiliarity with the wiring scheme and sought to clarify the wiring before he did anything to it. /QUOTE]

No he didn't, he removed the RCD protection from some of the circuits before coming on here to ask for help.

 

[davesparks]

By Marconi I contend that simple signage/notices at either end of the SWA would have brought it to the attention of all and thus removed the risk of unconsciously-incompetent folk doing something dangerous and or troublesome.

It would not remove the risk because the incompetant ignore any such signs. Just look at the number of idiots who drive past 'road ahead closed' and then the following 'road closed' sign only to have to turn around in someone's driveway and go back again for an idea of how widespread such ignorance is.
Putting 'do not remove' on a bonding connection doesn't stop people removing it.
Putting 'test every 3 months' on an enclosure with RCDs in it doesn't lead to people testing RCDs.
Putting 'emergency exit only, door alarmed' on a door doesn't stop people opening it
The more writing you put on something the less likely it is that someone will read it!