Sy cable question

[LeeH]

why can't you get a 3core and earth

Because that would make it a 4 core.

And using the third GY core as a phase conductor was my point. You cannot get a suitable 3 core SY for VSD applications for that reason.

 

[LeeH]

it's an exposed conductive part, therefore it must be earthed.

This.

And I think it's nuts saying it does not require it. And I just don't care if it's written in a book or not.

If that cable gets damaged that braid is likely to become live.

 

[westward10]

This.

And I think it's nuts saying it does not require it. And I just don't care if it's written in a book or not.

If that cable gets damaged that braid is likely to become live.

I am not saying yes or no to earthing it, merely where is the guidance. How are you judging its fault current carrying capabilities.

 

[Spoon]

I am not saying yes or no to earthing it, merely where is the guidance. How are you judging its fault current carrying capabilities.

I don't think anyone is using the braided screen as an earth cpc. They are just earthing the screen as its a sensible thing to do.

 

[westward10]

I don't think anyone is using the braided screen as an earth cpc. They are just earthing the screen as its a sensible thing to do.

I never suggested this, my point being if it is required to carry fault current due to cable damage where is the maths to assess its fault current carrying capabilities.

 

[Andy78]

I never suggested this, my point being if it is required to carry fault current due to cable damage where is the maths to assess its fault current carrying capabilities.

It would only need to be assessed for fault current carrying capability if used as a cpc surely ?
When earthed it is connected to cpcs whose capability for fault conditions has already been assessed and verified.

 

[Spoon]

I never suggested this, my point being if it is required to carry fault current due to cable damage where is the maths to assess its fault current carrying capabilities.

Sorry mate. I miss read your post.
I haven't seen anything for the fault current carrying capabilities of the screen.

 

[Wilko]

Hi - found this briefing note from BASEC, the official British Approval Service for Electric Cables (I think). They explain a bit more about the status of SY ++ .

Their last para is stark : "We therefore urge extreme caution in accepting the use of YY and SY cable types for UK fixed installations, and to contact the BCA on 0208 946 6978 ..."

BASEC - British Approvals Service for Cables - http://www.basec.org.uk/frontend/custom/print.cfm?part1=News&part2=Basec-News&part3=Be-Cautious-of-Midleading-Claims-Associated-with-YY-and-SY-cables&print=1

 

[Spoon]

Their last para is stark : "We therefore urge extreme caution in accepting the use of YY and SY cable types for UK fixed installations, and to contact the BCA on 0208 946 6978 ..."

What is classed as "fixed installations"?
I did see something on a NICEIC site stating a fixed installation is something that “would stay if the business moved.”

 

[LeeH]

I never suggested this, my point being if it is required to carry fault current due to cable damage where is the maths to assess its fault current carrying capabilities.

Do you asses this when earthing cable basket?

No, so I don't see the relevance of its ccc especially as it's not a cpc.

 

[Deleted member 93172]

We used to use this cable a lot in commercial situations for computor installations, the electrical consultants required the braid to be earthed at the distribution board end, because a capacitance can build up in the braid if not earthed down. And if more than one socket on a circuit the braid would be continued through to the last socket on the run.The braid was peeled back, sleeved with g/y and then lugged off and grounded at the distribution point, also large plastic stuffing glands were used for entry to enclosures.

 

[westward10]

It would only need to be assessed for fault current carrying capability if used as a cpc surely ?
When earthed it is connected to cpcs whose capability for fault conditions has already been assessed and verified.

People are quoting it should be earthed in case the cable gets damaged, say by a nail. What happens when the nail causes a short between a line conductor and the braid, fault current flows. So my point is can the braid carry the fault current and sufficient current without any deterioration of the braid to cause disconnection of the circuit. I am not suggesting its use as a cpc, but if it is earthed there is a possibility it may carry current under fault conditions.

 

[westward10]

Do you asses this when earthing cable basket?

No, so I don't see the relevance of its ccc especially as it's not a cpc.

That doesn't even warrant a reply.

 

[Darkwood]

I understand what you are saying @westward, and this is the very point I have been trying to make, if you wish to use SY then because it falls outside the BS7671 then its down to the installer to confirm if the braiding will be adequate to carry a fault current, the stuff I have used in the past has met this criteria as I have sourced the manufacturing info myself but I cannot say this is the same for every piece of SY out there, but I would say its best to earth than not to, given that even if it does take damage under a L/E fault then it should still trip the OCPD and investigation to why the circuit tripped should see it taken out of service anyway.

 

[andyb]

My take is that if the braid is an exposed conductive part then it will need earthing.
I suppose you could trim the braid and tape it so that it's not accessible when the circuit is live, then say it's not an exposed conductive part so doesn't require earthing. I don't know why anyone would do that, just earth it and be done with it.

 

[westward10]

I understand what you are saying @westward, and this is the very point I have been trying to make, if you wish to use SY then because it falls outside the BS7671 then its down to the installer to confirm if the braiding will be adequate to carry a fault current, the stuff I have used in the past has met this criteria as I have sourced the manufacturing info myself but I cannot say this is the same for every piece of SY out there, but I would say its best to earth than not to, given that even if it does take damage under a L/E fault then it should still trip the OCPD and investigation to why the circuit tripped should see it taken out of service anyway.

Yeah I know you get the drift, interesting you find it acceptable the braid may suffer damage under fault conditions, this is not acceptable and is the point I am trying to make.

 

[andyb]

Yeah I know you get the drift, interesting you find it acceptable the braid may suffer damage under fault conditions, this is not acceptable and is the point I am trying to make.

It's not really the point whether the braid will suffer damage, but that without earthing it a fault such as a nail penetrating the braid and live conductor will be live.
Not only that but the braid will be live along the whole length of the installation.

 

[westward10]

It's not really the point whether the braid will suffer damage, but that without earthing it a fault such as a nail penetrating the braid and live conductor will be live.
Not only that but the braid will be live along the whole length of the installation.

Isn't it, the braid should maintain it's integrity under fault conditions so the circuit disconnects whether the cable is no longer serviceable or not. It seems to be the case for all other circuit configuration unless SY is involved, just earth it job done no thought required.

 

[andyb]

Isn't it, the braid should maintain it's integrity under fault conditions so the circuit disconnects whether the cable is no longer serviceable or not. It seems to be the case for all other circuit configuration unless SY is involved, just earth it job done no thought required.

Defiantly not. If the braid is an exposed conductive part then it will need to satisfy the regs regarding disconnection time if a fault occurs. This may be earthing it, it may require an rcd. If you can't guarantee the disconnection time then it should not be used.
So you see, lots of thought.
Just ignoring it because you're not sure that it can maintain it's integrity under fault condition would come under the heading 'no thought required'

 

[westward10]

Defiantly not. If the braid is an exposed conductive part then it will need to satisfy the regs regarding disconnection time if a fault occurs. This may be earthing it, it may require an rcd. If you can't guarantee the disconnection time then it should not be used.
So you see, lots of thought.
Just ignoring it because you're not sure that it can maintain it's integrity under fault condition would come under the heading 'no thought required'

Whether it is an exposed conductive part is debatable. As a matter of interest how are you assessing the braids ability to carry fault current whilst suffering no thermal stress.