Wall Mounting A TV

[JohnW2]

No, it doesn't, but I didn't think a reference to HV would be correctly understood on a DIY forum.

Fair enough, but I rather suspect that 'MV' will be even less understood on a DIY forum!

'Safe to touch' does not necessarily equate to 'safe to remain in contact with for prolonged periods that can include temperature cycling and prolonged periods of vibration'.

Hmm, I think the barrels are being scraped again! I would imagine that any consequences of temperature cycling and vibration would be permanent - so are you suggesting that such a cable would become 'unsafe to touch' after it had been subjected to such environmental factors for a prolonged period?

Kind Regards, John

 

[securespark]

By HV do you mean >1000V?

 

[JohnW2]

By HV do you mean >1000V?

That is (now) the defintion (for AC, 1500V for DC) - officially "Normally greater than LV".

Kind Regards, John

 

[stillp]

Hmm, I think the barrels are being scraped again! I would imagine that any consequences of temperature cycling and vibration would be permanent - so are you suggesting that such a cable would become 'unsafe to touch' after it had been subjected to such environmental factors for a prolonged period?

No, I am stating that the requirement is made in consideration of the possible environmental influences, including time. We are not talking about a single cable, but about cables of different circuits in a single containment. I've personally seen an application where a thermocouple conductor (ELV) had shorted to a 400V conductor on a pump, causing a fire at the instrumentation panel. I've seen photos of a crane control panel in Rotterdam in which a LV conductor shorted to a 16kV conductor.

 

[JohnW2]

I've personally seen an application where a thermocouple conductor (ELV) had shorted to a 400V conductor on a pump, causing a fire at the instrumentation panel.

Through the insulation (and sheath, if any) of the ELV cable?

I've seen photos of a crane control panel in Rotterdam in which a LV conductor shorted to a 16kV conductor.

... and the corresponding question about that.

Kind Regards, John

 

[stillp]

Through the insulation (and sheath, if any) of the ELV cable?

Yes.

... and the corresponding question about that.

And the corresponding answer about that.

 

[JohnW2]

Through the insulation (and sheath, if any) of the ELV cable?

Yes.

I see. Maybe I need to clarify my question, then .....

Through the intact insulation (and sheath, if any) of the ELV cable?

Kind Regards, John

 

[stillp]

Depends what you mean by intact John. The conductors had not come into contact (at least on the one I saw) but the insulation had worn to the point where it was no longer able to withstand the voltage difference.
The consequences were that a voltage higher than intended was applied to the circuitry downstream of the fault, in this case an instrumentation panel that was not designed to see more than 24 V DC. In the Dutch case I believe it was a 110 V AC control panel that was connected by the fault to 16kV.

 

[JohnW2]

Depends what you mean by intact John. The conductors had not come into contact (at least on the one I saw) but the insulation had worn to the point where it was no longer able to withstand the voltage difference.

Hmmm. Presumably the insulation (and sheaths, if any) of both the 400V and the ELV cable must have worn to the point at which (what was left of) the insulation of the two of them combined was no longer able to withstand the voltage difference - which suggests that the insulation of the 400V cable alone was well on its way to not being able to withstand 400V (hence possibly 'not safe to touch').

We seem to be talking here about 'resistance to being worn away' and, given that that 'wearing away' is presumably a continuous and progressive process, it would eventually become a problem no matter what the 'voltage rating' of the cable carrying the lower voltage, wouldn't it?

If the insulation of the 400V conductor itself remained capable of 'withstanding 400V' then there would presumably not be a problem even if the outside of that cable touched an uninsulated ELV conductor, would there?

Of course, in the context of this thread we are talking about cables within trunking which would never be 'disturbed', so none of this 'wearing away' would be relevant. If the LV cable started life with insulation/sheathing "capable of withstanding 230V" (or whatever), that should remain the case 'indefinitely' and contact between the outside of that cable and even bare ELV conductors would never be a problem.

Kind Regards, John

 

[stillp]

We seem to be talking here about 'resistance to being worn away' and, given that that 'wearing away' is presumably a continuous and progressive process, it would eventually become a problem no matter what the 'voltage rating' of the cable carrying the lower voltage, wouldn't it?

Probably would. That's an argument against mixing voltages in the same containment, whatever the insulation rating.

 

[JohnW2]

Probably would. That's an argument against mixing voltages in the same containment, whatever the insulation rating.

It is - but that's not what we're discussing. As far as I am aware, BS7671 merely requires that all cables within the same containment are 'rated' for the maximum voltage present in any of the cables.

Kind Regards, John

 

[stillp]

Yes, it does. Perhaps that's not enough.

 

[JohnW2]

Yes, it does. Perhaps that's not enough.

Opinions will obviously vary about that.

For what it's worth, I've already expressed my ('inexpert') personal opinion in relation to domestic installations - namely that the requirements of BS7671 are already "more than enough". As I've said, my personal (again 'inexpert') opinion is that, in domestic situations, provided that each cable is adequately rated for the voltage it carries, any risk of 'interconnection' between the conductors of different cables is, in reality, negligible.

However, as I've said, opinions (even 'expert' ones) will undoubtedly vary.

Kind Regards, John

 

[stillp]

my personal opinion in relation to domestic installations - namely that the requirements of BS7671 are already "more than enough"

True, for domestic installations, but the scope of BS7671 extends to all electrical installations.

 

[stillp]

provided that each cable is adequately rated for the voltage it carries

I argued (unsuccessfully) in an IEC WG that each cable should have an insulation rating for the greatest voltage difference. Perhaps a greater problem with DC, but...