Acceptable to use earth in 3C+e to carry 12v DC? (read before replying!)

[Seb101]

I've got to spur in a transformer, it's to replace a mains-switched thermostat with a modern 12VDC 'smart' thermostat. The transformer is double insulated and the thermo is SELV.

I've got some 3C+E lying around. I want to use it to take 240V from the isolator switch on two of the conductors, to the transformer, and return 12VDC across the third conductor and earth.

So long as I sheath the earth wire so it is obviously NOT earth, is this acceptable?

 

[Taylortwocities]

NO!
You must not have 12v DC and 230V AC in the same cable! You must run a separate cable.

In any event, the cable must have its CPC connected to earth if it is carrying mains voltage.

 

[Seb101]

NO!
You must not have 12v DC and 230V AC in the same cable!

OK. 2 pieces of 2-core flex it is then....

 

[Seb101]

NO!
You must not have 12v DC and 230V AC in the same cable! You must run a separate cable.

In any event, the cable must have its CPC connected to earth if it is carrying mains voltage.

So - for my education. What's the regulation here?
Is it that 240V and 12V can't run in the same conductor bundle?
Is it that a CPC, if present, must be earthed?

Would using a single piece of 4-core flex (4 insulated conductors) 2 at 240VAC and 2 at 12VDC be acceptable (hypothetically)?

 

[winston1]

Would using a single piece of 4-core flex (4 insulated conductors) 2 at 240VAC and 2 at 12VDC be acceptable (hypothetically)?

I suggest you read post 2.

 

[bernardgreen]

Is it that a CPC, if present, must be earthed?

If there is mains voltage in a fixed cable then there must be CPC and that CPC must be earthed even if the equipment supplied is double insulated and doe not require an earth,

(4 insulated conductors) 2 at 240VAC and 2 at 12VDC be acceptable (hypothetically)?

No acceptable......... neither real nor hypothetical

 

[Seb101]

Is it that a CPC, if present, must be earthed?

If there is mains voltage in a fixed cable then there must be CPC and that CPC must be earthed even if the equipment supplied is double insulated and doe not require an earth,

(4 insulated conductors) 2 at 240VAC and 2 at 12VDC be acceptable (hypothetically)?

No acceptable......... neither real nor hypothetical

So what do you do with the flying earth wire at the double insulated appliance end? Surely it's more dangerous to have an unterminated earth tail that could potentially touch another terminal, than run a 30cm cable without an earth....

 

[bernardgreen]

In case it is mentioned

You can have 230 and 12 volts in the same "cable" but that cable is equivalent to twin and earth cable ( insulation and sheath ) for the 230 volt and cores for the ELV which are insulated and contained within a sheath to form a second cable. Then a third sheath covers the two other sheaths to make a single cable. Hence a bespoke cable used only in a few specific circumstances and best avoided.

 

[bernardgreen]

flying earth wire at the double insulated appliance end?

You terminate it in a terminal block.

 

[Taylortwocities]

OK. 2 pieces of 2-core flex it is then....

No, twin and earth (for the mains connection) 2-core flex for the 12v

 

[ban-all-sheds]

No acceptable......... neither real nor hypothetical

Why?

 

[ban-all-sheds]

In case it is mentioned

You can have 230 and 12 volts in the same "cable" but that cable is equivalent to twin and earth cable ( insulation and sheath ) for the 230 volt and cores for the ELV which are insulated and contained within a sheath to form a second cable. Then a third sheath covers the two other sheaths to make a single cable. Hence a bespoke cable used only in a few specific circumstances and best avoided.

The sheath is not insulation, it is mechanical protection.

Provided the individual cores are insulated for the highest voltage present you may have different voltages in the same containment system. i.e. the sheath.

 

[Taylortwocities]

Yes, but it is a requirement of a cable carrying (e.g.) 230v that the cores are double insulated. That's the core's individual insulation plus the outer sheath.

My understaniding is that if there are additional conductors carrying Band I then the DI requirement is not met.

 

[bernardgreen]

BAS I was basing the construction of bespoke cable on the way portmanteau cables were designed and assembled for various projects.

The inner sheaths are considered as part of the insulation beween LV and ELV ( in one case HV was also involved ) If all the ELV conductors had insulation thick enough to cope with the potential differences between ELV and LV then the overall cable size would be more than doubled.

 

[ericmark]

With the capacitive and inductive links between cores I can't see how one can ensure the voltage of the SELV cable remains below 50 volts from earth. You could likely use a FELV supply but not a SELV supply. (414.1.1)

414.4.1 refers to insulation and the insulation between cores of a triple and earth cable I don't think would comply. Protective screening is referred to which would stop the capacitive and inductive links between cores.

414.4.2 Protective separation of wiring, systems of SELV or PELV circuits from the live parts of other circuits. Which have at least basic insulation, shall be achieved by one of the following arrangements:
(i) SELV and PELV circuit conductors enclosed in a non-metallic sheath or insulating enclosure in addition to basic insulation
(ii) SELV and PELV circuit conductors separated from conductors of circuits at voltages higher than Band I by an earthed metallic sheath or earthed metallic screen
(iii) Circuit conductors at voltages higher than Band I may be contained in a multi-conductor cable or other grouping of conductors if the SELV and PELV conductors are insulated for the highest \ voltage present
(iv) The wiring systems of other circuits are in compliance with Regulation 412.2.4.l
(v) Physical separation.

NOTE 2: Requirements for separation and segregation in relation to safety services are given in BS 5266 and BS 5839.

It is quoted 100 mm separation for underground cables similar separation is also required above ground, I remember the fuss when we cut and removed all the alarm cables that had been placed on the LV cable tray the alarm installation company was told in no uncertain terms that was not acceptable.

As to use of FELV that is up for debate. As far as I can see if the ELV supply has one leg earthed then there is little likely hood of either leg voltage to earth raising above the 50 volt limit. I would be interested to see what others think of FELV use as I am not sure on this?