Type C Mcb

[Delboy]

Hi Folks, I have been asked to quote for an armoured supply down a garden to a hot tub, the manufacturers specify a type c mcb and the client wishes a separate twin socket outlet at the same position down the garden, my question is, can I run a normal twin s/s/o from a type c or do I need a bigger multi core swa to give separate supplies to the hot tub and the s/s/o? The installation is RCD protected, be good to hear your suggestions (I think!)

 

[LeckyBill]

is this a windup

 

[Richard Burns]

You are probably going to have Zs problems with a long cable going down a garden so it would be best to keep the current as low as possible so that you can have a higher Zs.
In that case it may be necessary to split the circuit or provide a fuse at the origin and a local C type for the hot tub (discrimination permitting).
However if your calculations come out OK then a type C is fine for a socket.

 

[Thefailsafe2012]

All depends on the Zs of the circuit, could be argued that as its RCD protected, if above the allowed Zs, then it'll be ok.

However personally I'd want to make sure the Zs was low enough to suit whatever rating of breaker is required without relying on the RCD


Have you got your cable size, length of run, MCB rating etc?

edit - what he said ^^

 

[Delboy]

Thanks Richard, have only ever used type c on motor circuits etc so was unsure of using one on a standard socket outlet, appreciate your comments.

 

[Delboy]

Its a 40m run with a 3kw load for the tub, the cable is surface mounted not dug in so my cable calcs made it a 2.5 for this increasing it to a 4mm with additional socket outlet

 

[Richard Burns]

All depends on the Zs of the circuit, could be argued that as its RCD protected, if above the allowed Zs, then it'll be ok.

However personally I'd want to make sure the Zs was low enough to suit whatever rating of breaker is required without relying on the RCD


Have you got your cable size, length of run, MCB rating etc?

edit - what he said ^^

I totally ignored the RCD protection bit, I must become more lax on my designs!

 

[Thefailsafe2012]

I totally ignored the RCD protection bit, I must become more lax on my designs!

Dont do it Burnsy, had a nasty experience with a proteus RCD masquerading as a mere main switch lol :cheesy:

 

[Guitarist]

What is the main earthing setup?

 

[Richard Burns]

Its a 40m run with a 3kw load for the tub, the cable is surface mounted not dug in so my cable calcs made it a 2.5 for this increasing it to a 4mm with additional socket outlet

Sounds like you are just about scraping through on volt drop and CCC if you were to use a 32A C type with a 26A design current and be OK on Zs if you have a Ze of less than 0.35.

Might be an idea to up it to 6mm to give some leeway, seeing I am not being lax now!

 

[Delboy]

Signing off now guys, thanks for your help, fairly painless! Guitarist it's a TNCS service with a 17th edition board, nite nite!

 

[telectrix]

agree with richard here, 6mm, which should give you a bit of leeway in case of the load being increased. you know what customers are. " oh, goody, here's a socket. plug this 3kW patio heater in. it's getting a bit chilly sitting here with the pimms".

 

[Engineer54]

If this Hot Tub is being positioned on bare ground/grass or concrete/slabs etc then you will also need to drive in a supplementary earth rod if your supply is PME/TNC-S!! Check your BGB!! lol!!!

Personally i've never come across a hot tub with such a low KW rating, i'd be installing a 6mm 3 core SWA as a matter of course for this installation.

 

[telectrix]

eng., i had a hot tub job to rewire a bit ago. was <3kW. it was installed with a 1.5mm SWA 'terminated' into a RCD plug. no earthing to the armour, cable had been cut at some point and jointed with a choc block wrapped in tape. wettter than a -----'s drawers. i replaced the SWA with 2.5mm, glanded into ad. box, then cores into DB.

 

[davethsparky]

If this Hot Tub is being positioned on bare ground/grass or concrete/slabs etc then you will also need to drive in a supplementary earth rod if your supply is PME/TNC-S!! Check your BGB!! lol!!!

Can you expand on this a bit please? I am doing a hot tub supply and garage supply install next week and thus far am planning to set up a seperate TT earth for the supplies as my instincts are telling me to ditch the PME for a hot tub.

Edit:

Insomnia is still bugging me so i've just read up on this in the BGB and guidance notes, references to this aren't all that obvious or big but ive found it. If i've read it right the requirement is for the addition of an earth electrode connected to the MET with a recommended Ra for the electrode of 20ohms.

I assume this prevents a dangerous potential rise in relation to true earth if a broken neutral occurs on the DNO network, and will result in a significant current flow through the rod as neutral currents are diverted. Is this in effect a bond to the local ground?

If my above assumption is correct would it not then be sensible to have this earth electrode installed for all PME supplies?

 

[Sintra]

It would make sense and is in fact done in some countries the closest being the Republic of Ireland where it is referred to as neutralising.

 

[Guitarist]

If this Hot Tub is being positioned on bare ground/grass or concrete/slabs etc then you will also need to drive in a supplementary earth rod if your supply is PME/TNC-S!! Check your BGB!! lol!!!

Personally i've never come across a hot tub with such a low KW rating, i'd be installing a 6mm 3 core SWA as a matter of course for this installation.

This is why I asked what the earthing was.

 

[Engineer54]

Can you expand on this a bit please? I am doing a hot tub supply and garage supply install next week and thus far am planning to set up a seperate TT earth for the supplies as my instincts are telling me to ditch the PME for a hot tub.

Edit:

Insomnia is still bugging me so i've just read up on this in the BGB and guidance notes, references to this aren't all that obvious or big but ive found it. If i've read it right the requirement is for the addition of an earth electrode connected to the MET with a recommended Ra for the electrode of 20ohms.

I assume this prevents a dangerous potential rise in relation to true earth if a broken neutral occurs on the DNO network, and will result in a significant current flow through the rod as neutral currents are diverted. Is this in effect a bond to the local ground?

If my above assumption is correct would it not then be sensible to have this earth electrode installed for all PME supplies?

That's right, the earth rod is additional or supplementary, rather than to replace the PME earth. As you say, it would indeed be sensible to have a localised earth electrode for all PME/TNC-S supplies. Not with a value of anywhere near 100/200 ohm Ra though lol!!. This is actually a requirement in many European countries where PME supplies are provided. In effect it's another point of the protective multiple earthing system, but using the MET for the N-E connection. I'm not so sure that the additional rod would be best connected to the MET, in this case, ...i'd connect direct to the Hot Tub's main earthing terminal.... Does the hot tub have any integral RCD protection??

As for the recommended 20 Ohm Ra, like any earth electrode system, always try and get as low a value as you can, rather than just aiming for the arbitrary max value..

 

[davethsparky]

I'm not quite following the logic of bot connecting it to the MET here, would connecting it directly to the hot-tub not cause a diverted current to flow through the hot tub's CPC to the earth electrode? Especially if the dreaded lost Neutral fault was to occur on the PME supply? Bearing in mind the Ib for this hot tub is only 25A so the cable an't going to be very big.

 

[Geoffsd]

If the rod is 'additional' to the PME earthing would it then not just be an extraneous-conductive-part and require main bonding to the MET.

In effect bonding the ground.

 

[Guitarist]

If you were to rod the hot tub's panel then it would have to be either isolated from the supply earth by not connecting the cable's earth at the hot tub end (creating its own TT), or you'd run a 10mm bonding cable back to the supply MET.

 

[Engineer54]

I'm thinking more towards this tub being located/positioned on bare ground or grass. I'm not really that bothered about broken neutrals on a TNC-S/PME supply, as the chance of ever seeing that, is going to be remote to say the least!! As stated i'd be installing a 3 core 6mm SWA supply cable out to this hot tub, which would provide an ample CSA of copper CPC going back to the MET. I'd probably also use a 6mm earthing conductor from the hot tubs local earth rod too!!

Bare feet on wet earth and grass, or on wet concrete is another matter altogether though, ...now you're in the realms of potential differences and step voltages!!

 

[Engineer54]

If you were to rod the hot tub's panel then it would have to be either isolated from the supply earth by not connecting the cable's earth at the hot tub end (creating its own TT), or you'd run a 10mm bonding cable back to the supply MET.

I'd be happy to have either, (if the TT was of a low value). In this case, I reckon 6 mm of copper, will do an adequate job of protection, even if it's not exactly what the BGB say's!! lol!!

 

[Guitarist]

I'd be happy to have either, (if the TT was of a low value). In this case, I reckon 6 mm of copper, will do an adequate job of protection, even if it's not exactly what the BGB say's!! lol!!

I agree Eng. Just quoting regs really for the op

 

[Engineer54]

I agree Eng. Just quoting regs really for the op

Actually, seen more than one manufacturer clearly state, to connect a supplementary earth rod (and sometimes supplied) to the Hot Tub/ Jacuzzi control panel.