AC mains and low voltage DC in same cable?

[Darkwood]

I would assume so, as the conductors used for the Ethernet are the ones already being used for the 230V.

LOL - your been a tad cheeky here, injecting a waveform signal into a cable been used is obviously not what is been discussed here and works off totally different technology

 

[Deleted member 26818]

Just wondering why there has to be screening between cores in some multicore cables, when it's possible to use the same cores for both ac and dc?

 

[davesparks]

How will you get induced voltage on the dc circuit if the cores are insulated to the highest voltage present i.e. 230volts?

Induction relies on the cores being insulated from one another, otherwise it wouldn't be an induced voltage, just a direct connection.

 

[Darkwood]

Just wondering why there has to be screening between cores in some multicore cables, when it's possible to use the same cores for both ac and dc?

It is probably because of the issues a fault could cause IE in a damaged cable, of that regulation the exception no' 6 specifically states Earth screening to the same level of the highest rating of the largest conductor thus implies it is been implemented as an earthed barrier, as it is discussing LV and ELV here (AC/DC) not actually been the issue then it comes down to a few factors, consider how DC or ELV AC is generated of our LV supply, it's common but not exclusive for it to have galvanic isolation, this would allow for potential dangerous voltages to exist on ELV gear without fusing, some of the cheaper DC motor controllers I use do not possess galvanic isolation and this leaves a floating voltage at LV level which can exist on the controls, not a issue until you start connection 0-10v speed controls on them and the user might have wet hands... I have to ensure I fit in a galvanic isolation unit in the circuit for user safety, the same principles could happen to a damaged cable carrying both LV and ELV without an earthed screen between them.

The crucial factor here is what the OP is connecting here, what the DC is for and how it is generated, it still doesn't give room for any excursions from the regulations although you could if it is shown to not be an issue of any kind, any regulation in the BS7671 is advisory although you need to ensure you have a lot of paperwork to justify any such breaches of regulations.

 

[Wilko]

As I interpret it that is for the same wiring system, they have made a specific regulation exemption No' 6 for multicore cable as that does not fall under the term same wiring system due to the fact its a cable and not multiple cables or cores in a wiring system.

Hi - I'm going to stick my neck for purposes of a good discussion, and not saying I'd like to run 12V in the same multicore cable as 400V .

Reg 528.1, intro says no mixing Band 1 and Band 2 unless one of the 6 exemptions applies. Understood. I take it on board about the 6th method, but if we read it as a specific method for multicore cable, what about the 2nd method? Where it also says "each conductor of a multicore cable". Doesn't that mean there are 2 exemption methods for multicore cable? I haven't spotted why 6 should be read as separate from a wiring system but 2 should not

 

[Ian1981]

Hi - I'm going to stick my neck for purposes of a good discussion, and not saying I'd like to run 12V in the same multicore cable as 400V .

Reg 528.1, intro says no mixing Band 1 and Band 2 unless one of the 6 exemptions applies. Understood. I take it on board about the 6th method, but if we read it as a specific method for multicore cable, what about the 2nd method? Where it also says "each conductor of a multicore cable". Doesn't that mean there are 2 exemption methods for multicore cable? I haven't spotted why 6 should be read as separate from a wiring system but 2 should not

It mentions wiring systems so if I read it again to me it's saying that if I have singles in conduit I can run band 1 and band 2 in the same conduit wiring system if each conductor is insulated to the highest voltage present i.e. 230 v.
Is a multicore cable then a wiring system in its self is the question?
If so then it's allowed
I'm kinda going back on forward on this one.
Anyone else?

 

[Ian1981]

Actually to me option 6 is saying if your cable isn't insulated against the highest voltage present on each conductor then that needs to be employed with earth screening.

 

[Darkwood]

Well in honesty I did say IMHO, yes I missed that part in point II @Wilko - and agree with both you and @Ian1981 on that.
I work on machinery to differing reg's and the reason for my take on this is safety under fault conditions, I see a big flaw in this regulation and I may raise it with the IET myself, but it would seem simply having the insulation value to the highest conductor would suffice, I would still consider the implications of the LV crossing over to the ELV and the potential risks associated with it if it were to occur, fusing alone isn't always a sufficient, like I said I said before, if DC is generated and isolated from the LV side in its generation then this could potentially create a accident waiting to happen.

 

[Darkwood]

With regards to wiring systems, usually by the nature of the wiring system you are afforded extra mechanical protection, be it trunking, conduit etc and as long as it is suitable for the environment then it should give a good serviceable life protecting the cables inside, a multi-core cable however could be just a flex and is more likely to be at risk from damage so I would argue that extra consideration be taken when choosing this method although the OP is discussing SWA so mechanical protection is somewhat afforded.

 

[Ian1981]

Well in honesty I did say IMHO, yes I missed that part in point II @Wilko - and agree with both you and @Ian1981 on that.
I work on machinery to differing reg's and the reason for my take on this is safety under fault conditions, I see a big flaw in this regulation and I may raise it with the IET myself, but it would seem simply having the insulation value to the highest conductor would suffice, I would still consider the implications of the LV crossing over to the ELV and the potential risks associated with it if it were to occur, fusing alone isn't always a sufficient, like I said I said before, if DC is generated and isolated from the LV side in its generation then this could potentially create a accident waiting to happen.

It's been a good discussion tho in which the forum is all about.
No name calling either

 

[rod]

Many thanks for your replies all. This was to run LED markers on external posts using the spare capacity in the armoured power-cable, although perfectly capable from an insulation point of view it felt wrong mixing these voltages in the same cable, hence my post. This would have saved many transformers and resultant bulky housings at each location. I will not run the 12vdc.

 

[Wilko]

... I would still consider the implications of the LV crossing over to the ELV and the potential risks associated with it if it were to occur, fusing alone isn't always a sufficient, like I said I said before, if DC is generated and isolated from the LV side in its generation then this could potentially create a accident waiting to happen.

Yes I agree, the ELV may be very basic ("it's only 12V...") and probably shouldn't be in the same jb containment (?) which would likely happen if it was within a multicore swa. Perhaps for ELV control signals for a machine (that's being powered by the other cores of the swa, say) then it could be ok per 2nd option ?

 

[Deleted member 26818]

Over the years, I have been tasked with wiring a few Dali type lighting systems using 5 or 7 core cables.
3 or 4 cores for power and 2 cores for data.
The power is LV ac and the data ELV dc.
Cables have been either flex or SWA depending whether they are internal or external.
None of the cables have had internal screening between the LV and ELV.

 

[davesparks]

Over the years, I have been tasked with wiring a few Dali type lighting systems using 5 or 7 core cables.
3 or 4 cores for power and 2 cores for data.
The power is LV ac and the data ELV dc.
Cables have been either flex or SWA depending whether they are internal or external.
None of the cables have had internal screening between the LV and ELV.

Yes, I have done similar and as far as I know this is perfectly compliant (conductors insulated to highest voltage)
And presumably the ELV data/control circuitry is designed such that it immune to 'noise' or other interference from the AC.

 

[Lucien Nunes]

One other consideration is the method of protection of the two circuits. The LV circuit is likely to be TN mains relying on ADS, while the ELV circuit might be expected to be electrically separated, perhaps SELV, and requiring more than simple separation at the terminations. A fault from the LV to the ELV such as tracking between terminals in a JB might not be detected and might persist. Within the cable, the ELV circuit is still insulated up to LV from the outside, so it doesn't create a shock risk there, but unless all the ELV fittings are insulated for LV they would pose a shock risk.

Re. sending ethernet over mains cables, this is a different scenario because it's not an ethernet 'circuit' and a power circuit sharing a cable, it's just the power circuit with the data modulated onto it electronically. The sender and receiver provide the electrical separation and the modulation scheme is designed to be tolerant of interference. You would not get the ethernet physical layer itself to go very far down SWA!

Data transmission systems designed to be integrated with LV AC power are usually differential, e.g. DALI / DMX /RS485 or current-driven if analogue e.g. 4-20mA which makes them particularly resistant to interference. Twisted-pair ethernet is also differential but it goes so much faster that the cable's inter-pair crosstalk is much more of a limiting factor than interference pickup from other circuits.

 

[tom porter]

How will you get induced voltage on the dc circuit if the cores are insulated to the highest voltage present i.e. 230volts?

do you not know what an induced voltage is?

 

[redbarnman]

What this discussion shows is how you can write rules that still don't make a requirement clear and unambiguous. An extension of this would mean no ELV (12 or 24v AC/DC) or signal cables in the same trunking or conduit as LV (mains). It may be undesirable because induction will put a small "50Hz hum" signal on the ELV and when mains switching of inductive elements takes place e.g washing machine motors or a resistive spike as an elecric kettle is turned on a small signal spike will be induced in the ELV circuit. Induction effects can be minimised by using twisted pair as in CAT5 etc. Seems to me there is no issue with using LNE and DC1 and DC2 in a 5 core cable so long as you can deal with the induced voltages within the ELV equipment.