Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 14 replies
  • Subscribers 30 subscribers
  • Views 1724 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Using BS8436 cable - Thinking check

Scrambled

Good morning everyone, its that time again where if you could spare five minutes of your expertise I would be much obliged!

I currently have to install cabling and controls for a new retrofit underfloor heating system. This system is low profile and involves the pipework laid on a (appx 20mm deep) mat. The existing floor construction is concrete. All pipework will then be set in a deep base levelling compound prior to full tiling of the floor.

The house is fully decorated and the client is unwilling to disturb decoration unless absolutely necessary. It has been suggested to remove coving and run cabling behind to thermostats but this has met firm opposition. There is no access from above other than this.

My current thinking - and it is this that I would appreciate your input on/abuse if you think it's unacceptable :)…

To install 1.5mm BS8436 cable within 16mm flexible conduit in screed where necessary and otherwise behind skirting boards which are being replaced in the course of works. Where in floor (in my created building void) the conduit will be 100mm away from the central heating loops but would inevitably be within the thermal mass of the floor. It will be installed away from where any future installation of carpet would necessitate gripper rods to be nailed down as far as practicable.

Conservatively I have calculated the following derating factors which I hope will be more than conservative enough to make sure Iz is within tolerance for my protective device.

Table 4D2A 1.5mm2 Cable Reference Method A (worst case scenario conservatively selected as more likely to be reference method b). Four core cable rating (three phase (again going conservative to allow for the switch line). Current carrying capacity: 13 Amps At an ambient temperature of 30 degrees

13 x 0.50 = 6.5 amps  (derating for a 60 degree ambient temp even though specification says 45-50 degrees for the heating loop water temperature and the ambient temperature of the floor where the cables will be will almost certainly be far below this as they will not be up against them.

6.5 x 0.79 = 5.135  (derating for three grouped cables. Highly unlikely to be necessary (but just in case). These cables are for thermostats only so are almost certainly going to be carrying less than ⅓ grouped current rating.

5.135 x 0.65 = 3.33 amps (additional (bonus) derating for unventilated cabling system as per notes [2 layers of cable - unventilated]

I'm going to use a 3A type B MCB (class 3) at the manifold which will probably also act as the isolator for the central heating controls. 

Other than being super cautious can you guys see any wrong-think in my workings - would there be anything else you would consider?

This is the first time I've used BS8436 cable and primarily it is being used to get around the cable not being in permitted zones/at depth within the floor and walls. I've read the guidance with regard to using type B class 3 MCB's at a fault current <5kA which is what will be happening in this installation. I have noticed some suppliers give a maximum energy let through for the protective device (42000A2S) but annoyingly the particular supplier (Atom) that make the cable I have been given do not - I've asked them but am not holding my breath. Do you know whether this maximum energy let through is built into the product standard by any chance. The energy let through of the 3A type B MCB (and probably the 16A type B at the origin of the circuit) is almost certainly going to be far below 42000A2S within an installation with a PEFC <3kA - do I need to look any further into this or is the use of type B MCB's just a ‘given’ with this type of cable.

Hopefully you haven't all nodded off during that. Thanks in advance for any help and advice you can give.

 

 

 

 

 

Parents
  • 0

    Thanks both. In fairness Atom did come back to me just with an excerpt from the British Standard which mentions using a CPD with an energy let through <42000A2S.

    c85f9773879875233aab6dee78819c13-original-image.png

    I have seen this figure quoted in other manufacturers literature as well. I have the energy let through data for a B6 MK Sentry (they make a 3A type b as well) which puts the energy let through at 3kA at around 10000A2S which as you said Andy would be fine for a 1mm CPC. What is turning me about a little bit is that the test procedure doesn't seem to make an allowance for a low impedance/high fault current short. Im just struggling as to how I as the designer can ensure fault protection at all points in the circuit or will I have achieved this by virtue of meeting disconnection times and following the british standard? It would seem impractical to use this cable if it can only protect against a short to a maximum current of 100/160 amps!

    It makes sense the consideration that we're not necessarily protecting the pvc insulation but rather ensuring that the circuit can disconnect and I suppose at the point in time the cable gets pinned it is knackered but yes the thought of the foil frying post penetration and then the circuit being re-energised is not a comforting one. 

    Where did you obtain the information about the K2S2 of the foil being generally greater than that of a 1mm copper cpc Andy? I'd be happy my 3A MCB would take care of any drama if this were the case :)

     

    Thanks again chaps.

     

     

Reply
  • 0

    Thanks both. In fairness Atom did come back to me just with an excerpt from the British Standard which mentions using a CPD with an energy let through <42000A2S.

    c85f9773879875233aab6dee78819c13-original-image.png

    I have seen this figure quoted in other manufacturers literature as well. I have the energy let through data for a B6 MK Sentry (they make a 3A type b as well) which puts the energy let through at 3kA at around 10000A2S which as you said Andy would be fine for a 1mm CPC. What is turning me about a little bit is that the test procedure doesn't seem to make an allowance for a low impedance/high fault current short. Im just struggling as to how I as the designer can ensure fault protection at all points in the circuit or will I have achieved this by virtue of meeting disconnection times and following the british standard? It would seem impractical to use this cable if it can only protect against a short to a maximum current of 100/160 amps!

    It makes sense the consideration that we're not necessarily protecting the pvc insulation but rather ensuring that the circuit can disconnect and I suppose at the point in time the cable gets pinned it is knackered but yes the thought of the foil frying post penetration and then the circuit being re-energised is not a comforting one. 

    Where did you obtain the information about the K2S2 of the foil being generally greater than that of a 1mm copper cpc Andy? I'd be happy my 3A MCB would take care of any drama if this were the case :)

     

    Thanks again chaps.

     

     

Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines