cable suitability for dc

[davetheglitz]

Sorry Mr Newman - I know this isn't where you want the thread to go - but I would like to know why - from the basic physics - a 600vac SWA would not suitable for 800v dc. The peak to peak voltage on a 600vac cable is in the order of 900v - which is what the insulation would need to withstand. If the cable was for 3 phase this would be more. We happily bung 500vdc down our 230v twin and earth to test insulation - isn't this the same case?

 

[Mr Smith]

following yet another thread having to be addressed by the moderators

and mr newman having to start again

perhaps separating the discussions will be advantageous

be warned

mr smith is unhappy with the way the thread is railroaded off the original topic

anyone not using this

or another

thread for side discussions

will incur infractions

thank you

mr smith

 

[steptoe]

Doesnt DC cable need to be DI ?

 

[kme]

As far as I`m aware.

Plus - two other considerations.

1. What do you do with the armour? If you leave it floating, will it pick up enough capacitative charge to have a standing voltage on it? And what woulfd that be referenced to? If you earth it, you`ve introduced a reference that has no correlation to the DC; unless one side of the DC is earthed.........can of worms time!!!!!

2. Not everyone is conversant with PV systems - someone could see and operate all the AC isolation devices, and the DC iso at the inverter end, then have a shock from the DC, as they wouldn`t expect it to be there.

It could

 

[davetheglitz]

Doesnt DC cable need to be DI ?

Sorry Steps - billy basic stuff here - what's DI?

 

[ivor dispatch]

Sorry Steps - billy basic stuff here - what's DI?

I think he means double insulation, the DTI guide has a list of cables suitable for DC and swa is there best suited for main dc cable not behind modules

 

[davetheglitz]

So why would DC cables need to be double insulated? Is this the same argument as meter tails being sheathed/DI? The counter argument for this is SWA is used for submains.

1. What do you do with the armour? If you leave it floating, will it pick up enough capacitative charge to have a standing voltage on it? And what woulfd that be referenced to? If you earth it, you`ve introduced a reference that has no correlation to the DC; unless one side of the DC is earthed.........can of worms time!!!!!

On virtually all amplifiers and appliances the 0v dc from the power supply is connected to earth/chassis. If earthing of the cable is at one end only there are no earth loops. I don't know inverters - but if the same principles are involved why is this a can of worms?

2. Not everyone is conversant with PV systems - someone could see and operate all the AC isolation devices, and the DC iso at the inverter end, then have a shock from the DC, as they wouldn`t expect it to be there.

In this case there should also be an isolator at the source end and a big note on the inverter. However is this any different to any PV installation? Someone unfamiliar with PV could cut into the DC cable on any installation.

Sorry Guys - I'm just looking from the physics side rather than the regs.

 

[Manator]

Dave, if you remove the PV out of the equation and start to think about the physical differences between a.c. and d.c. then you are half way there.

There are many problems, some have been highlighted by Steptoe and KME above. The problem I see is that when PV installations began to hit the market basic d.c. voltage requirements and behaviour was some how ignored or most probably overlooked.

I have never done any PV installs but know enough knowledge to know that I, from what has been discussed here can see an ignorance of basic electrical knowledge, not intentional, but inbuilt into what appears to have gone before.

You should never treat d.c. the same as a.c. because they are infinitely different.

 

[davetheglitz]

You should never treat d.c. the same as a.c. because they are infinitely different.

Of course ac and dc are different - but within the context of cable design it strikes me that DC is considerably easier - no reactance to worry about - no transients - no peak voltages, no phase shifts so ultimately you come down to series and parallel resistance.

Canoeboy - what an excellent post - answered all of my points brilliantly!

 

[green-hornet]

All dc voltages should be kept below 12v until they know the difference.

Davetheglitz excellent questions mate, at least you are willing to sensibly debate a grey area.

Me? I would not really like to use multicore SWA cable for such large voltages of dc currents. I am however willing to learn if anyone can say this is safe to use "providing the design is correct"

 

[green-hornet]

I am actually surprised, seeing that the potential dangers with the steel armour could present a floating hazard, I did notice 80 degrees, so does that mean all accessories should be rated for this?

 

[Jono Pashley]

afaik the 80 degrees thing is about the temperature withstand of cables, not the operating temperature of the core. I could however be totally wrong.

Yes the DTI is a guide but for the purposes of MCS certain items within are mandatory and others expected unless theres good reason to do it a different way.

On another note ive never liked running the dc cables without additional mechanical protection even with labels every meter. Last one i did was all done in flexi conduit felt much happier with that.

 

[Sidewinder]

Whilst "BS standard" 5467, 6346, & 6724 do allow the use of SWA for d.c. the standard requires that the cable is verified for use at the peak d.c.voltage levels experienced in the event of a fault.

If this is not given by the cable maker & the equipment maker then it is down to the installer to verify this.

With the data that is available it is doubtful that this can be ascertained.

Remembering that "normal" SWA is only rated at 600/1000V.

At 800V d.c. you are very close to the upper limit, and above the lower one.

Thus care is required in the application.

Without all of the relevant technical data, it is doubtful that the cable can be ascertained as suitable...