mixed cables in conduit

[bernardgreen]

from https://uk.rs-online.com/web/generalDisplay.html?id=ideas-and-advice/telephone-cable-guide

BT Specification
Here in the UK, BT has established a number of commonly used telephone cable specifications. These are known as the CW1308 standard.

Cables made to the BT specification typically feature paired 0.5mm insulated copper conductors, but the number can vary hugely, from as little as 2 to as many as 200. The insulating sheath is normally PVC, but you may also see:

• LSF (low smoke and fume). This refers to the insulation’s resistance to fire. Conventional PVC emits thick black smoke and hazardous fumes when it burns or is exposed to high temperatures and this can interfere with attempts to escape in the early stages of a fire. LSF has been formulated to do neither
  
• LSOH or LSZH (low smoke zero halogen). Hydrogen chloride - a halogen, or salt-producing chemical element - is another dangerous gas emitted by some types of PVC when it burns or melts. It can cause breathing difficulties and lung damage if inhaled or skin burns if touched. If mixed with water it will form hydrochloric acid. LSOH insulation will emit neither fumes nor halogen in the event of a fire
  

A variation of the standard, the CW1308B, has reinforced insulation for use both outdoors and indoors. This resists not only weather but also UV (ultraviolet) light.

The CW1128 is an additional BT telephone cable specification. This is designed solely for outdoor use, with reinforced armoured models suitable for use below ground.

I am told that the insulation on CW1308B is rated at 600 Volts but have yet to see that in print.

 

[RandomGrinch]

I am told that the insulation on CW1308B is rated at 600 Volts but have yet to see that in print

Similar to the CAT6 already suggested (some CAT6 is even rated to carry 300V DC).
But these are 'proper' phone cables, not the cheap 'Poundshop' extensions!

 

[SUNRAY]

A variation of the standard, the CW1308B, has reinforced insulation for use both outdoors and indoors. This resists not only weather but also UV (ultraviolet) light.

The CW1128 is an additional BT telephone cable specification. This is designed solely for outdoor use, with reinforced armoured models suitable for use below ground.

I am told that the insulation on CW1308B is rated at 600 Volts but have yet to see that in print.

I have looked and not been able to find that, however I have used CAT5E external grade cable marked as 300/500V.

Yes, I totally agree
Although the use of singles for the mains with one less layer of insulation and a flimsy telephone wire may make the risk an infinitesimally small amount higher!

The thing about running a low level signal cable in with singles is the potential additional interference induced.

 

[chivers67]

In fact John i may have a length of screened Cat6 that could be used and I'd post it to Glasgow if you want.

 

[securespark]

From the POV of a professional electrician who started in the business coming up to 39 years ago, it isn't the right thing to do.

Why don't you ring up the IET tech helpline and discuss these issues you have?
You will get a better insight into them having done that.

 

[JohnW2]

Don't side-step the question. ... You know very well in this case we are talking about the installation of Band I and Band II cables together in the same cable enclosure.

Fair enough, but if that was what you were referring to, you really should have quoted my comments about that, not those about "infinitesimally small increases in near-zero risks" Anyway, to be clear about my position ...

Despite what I go on to say below, in the interests of 'decency' I would not put two or more cables in the same conduit/whatever if I did not believe that all has adequate insulation to cope with the highest voltage present in any of the cables. As I implied, the problem is with 'the ratings' which, for whatever reason (I guess because they 'do not need to') do not reflect the true voltage-withstanding capabilities of the cable. The reality is that the properties of PVC are such that to not be able to withstand 230V, it would have to be impractically thin, and liable to mechanical disruption.

In any event, the regs about this are not particularly rational, and I am a great believer in common sense. We are talking about mains-carrying T+E and a telephone cable within the same plastic conduit. The insulation and sheathing of the T+E itself is deemed to be such that it is safe to touch the outer surface of the sheathing, and also safe for that sheathing to touch 'anything' credible (including earthed things) - it therefore would seem to follow that it would/should also be considered safe for the outer sheath of the T+E to be in contact with even bare conductors carrying ELV, let alone insulated and sheathed ones. With insulated & sheathed T+E and insulated & sheathed telephone (or whatever) cable, it would require FOUR faults for conductors of one cable to come into contact with conductors of the other.

I think there is also inconsistency Some people get concerned about (insulated and sheathed) conductors carrying LV and ELV being in the same conduit/ducting/trunking/whatever if the cables are not all 'rated' for LV, but they don't seem to worry about situations in which items of equipment often contain (and often only 'single-insulated') conductors carrying both LV and ELV, with the latter not 'rated' for LV. Nor do I hear it being suggested that the sheaths of cables carrying LV and ELV must not be allowed to come into contact unless the ELV ones are 'rated for LV'.

Kind Regards, John

 

[JohnW2]

From the POV of a professional electrician who started in the business coming up to 39 years ago, it isn't the right thing to do.

I respect your principles but ...

... do you take precautions to ensure that the (outsides of the sheathing of) the 'mains lead' and antenna/HDMI/Ethernet/USB/whatever leads connected to your TV never come in contact with one another?

Kind Regards, John

 

[SUNRAY]

From the point of view of someone who has been actively engaged with low level signals for over 60 years (OK I'm including the time I was a todler and helping daddy) I'll say there is a lot of ignorance of what does and what doesn't work and also what the regs state.

If all of the cables are insulated to the highest voltage within the enclosure the regs are satisfied as far as the safety part is concerned.

Whether interference is likely is a different thing entirely.

I've lost count of the number of times I've found flexible conduit containing low level signal cables inside trunking otherwise containing mains cabling, making it regs compatible. However I've been called in on several ocassions as there is interference, this is most commonly due to unbalanced signals and less commonly due to mains singles.

As to asking the IET I was involved with intstalling a new AV system in the IET in Savoy Place about 20 years ago, we manufactured connecting plates containing audio, video and control sockets and unusually also mains sockets. We included metal flexible conduit for the mains cabling to totally segregate it from the signal cables. To say it was a PITA in the limited space is an understatement and then at handover to the client (IET but may have still been IEE then?)they inspected and asked why we'd run the mains flex in conduit.

 

[JohnW2]

If all of the cables are insulated to the highest voltage within the enclosure the regs are satisfied as far as the safety part is concerned.

... agreed - but, as I've said, the problem is that many signal/commons/whatever cables have very low claimed 'voltage ratings', despite the fact that they would (in my opinion) be perfectly capable of withstanding the highest voltage in any of the cables.

Also, as I've said, if it is considered safe for the outer of the sheath of, say, T+E to be touched by people, or to touch anything else, why is there necessarily a ('safety-based') requirement for an 'ELV' cable with which it may come into contact to have any insulation at all?

Whilst I respect secure's wish to fully comply with regs and MIs, I do think that an element of common sense is required - particularly given that, as you say, the 'non-compliant' practice we are discussing is incredibly prevalent in the real world.

Kind Regards, John

 

[SUNRAY]

I respect your principles but ...

... do you take precautions to ensure that the (outsides of the sheathing of) the 'mains lead' and antenna/HDMI/Ethernet/USB/whatever leads connected to your TV never come in contact with one another?

Kind Regards, John

one only has to look at the rear of the average surround sound/TV/PC setup to see that no such precautions are put in place or ever thought about.

For those involved with public entertainment we can quote massive bundles of mixed cables dumped on top of each other without any consideration and those may include microphones (~0.1μV audio or radio aerial), triac dimmed multi KW lights (Many channels of chopped 2-30A or more), MVA feeds from generators, video and digital data to list just 5 of the multitude of services and cable types likely to be found in such situations. sometimes big bundles (and on big jobs the bundles can be massive {on one job there was a doorway filled to about halfway deep}) can run a long way in very close proximity.
All of this is expected to work with no problems with no segregation.

 

[SUNRAY]

... agreed - but, as I've said, the problem is that many signal/commons/whatever cables have very low claimed 'voltage ratings', despite the fact that they would (in my opinion) be perfectly capable of withstanding the highest voltage in any of the cables.

Also, as I've said, if it is considered safe for the outer of the sheath of, say, T+E to be touched by people, or to touch anything else, why is there necessarily a ('safety-based') requirement for an 'ELV' cable with which it may come into contact to have any insulation at all?

Whilst I respect secure's wish to fully comply with regs and MIs, I do think that an element of common sense is required - particularly given that, as you say, the 'non-compliant' practice we are discussing is incredibly prevalent in the real world.

Kind Regards, John

Agreed... my #25 crossed in the post

 

[JohnW2]

I am told that the insulation on CW1308B is rated at 600 Volts but have yet to see that in print.

I've just looked at lot of specification/data sheets for CW1308 / CW1308B cables and somewhat surprisingly (perhaps because of the intended application), I have found no explicit mentions of 'voltage rating'. However, I did find this ...

Those criteria would be regarded as a 'pass' for any cable carrying LV in an electrical installation - so, if that is typical/true of all CW1308 cables, we are perhaps discussing a non-problem?

Kind Regards, John

 

[JohnW2]

one only has to look at the rear of the average surround sound/TV/PC setup to see that no such precautions are put in place or ever thought about.

Quite. My question was obviously essentially rhetorical.

Kind Regards, John

 

[SUNRAY]

I've just looked at lot of specification/data sheets for CW1308 / CW1308B cables and somewhat surprisingly (perhaps because of the intended application), I have found no explicit mentions of 'voltage rating'. However, I did find this ...
View attachment 254848
Those criteria would be regarded as a 'pass' for any cable carrying LV in an electrical installation - so, if that is typical/true of all CW1308 cables, we are perhaps discussing a non-problem?

Kind Regards, John

That would seem to provide suitable for mixed use, however the test figure and the specified figure are not the same thing.
One only has to compare with climbing and lifting equpment and the required test methods and figures.
That said it's a very different safety margin requirement.

 

[JohnW2]

That would seem to provide suitable for mixed use, however the test figure and the specified figure are not the same thing.

Agreed - but, as I said, the specifications / data sheets seem uniformly consistent in not 'specifying' any figure for 'voltage rating'. In the absence of that, I think all one can do is draw one's own conclusions from the test which at least one 'specification' says that the cable has to pass.

Kind Regards, John