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Using BS8436 cable - Thinking check

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 mapj1 over 3 years ago

This stuff then I think the ‘short circuit current’ column under the tab for the current rating is fantasy or in the wrong units. Use your own analysis.

Should meet the regs routed out of zone, and if it gets hit, well the RCD will go off, that is why we have it.

It is unusual, and I'd be minded to take photos or make a sketch and leave a copy in an envelope by the CU - it will not be what the next person to look at it will be expecting, make life easy for your future self and avoid them ringing you up to say it is an EICR fail when it isn't..

If you are really worried, what is the PSSC, and do you know the make of the MCB /RCD - quite often they publish their own disconnection curves which are quite a bit faster than the minimum, and you can then be reassured that you have some margin.

Mike

Those tests

the BS standard is:

Voltage rating: 300/500v
Range: 1.0mm to 4.0mm2, 2c 3c & 4c plus earth.

Three principle performance criteria called up in BS 8436:2011 are:

Nail Test:	A 0.9kg a weight dropped from a height of 400mm driving a steel nail through the cable, the cable is deemed to have passed if it operates up to a 32A Type B MCB in six consecutive valid strikes.

Impact test:	A 0.5kg chisel-edged weight is dropped from a height of 250mm and the cable shall maintain its electrical integrity. The same is done with a radiused weight. The test is carried out at 20° C.

Water immersion test:	A 20m sample of cable is immersed into water at 20° C for 24 hours. The cable’s electrical integrity is tested following this.

As an aside if you want a better cable - well one that is rated for a higher voltage, it is worth looking for dual rated Irish and UK cable.

IS273:

An existing Irish standard that was modified to include cable failing safe under nail penetration, this standard addition was based on BS8436, but has a number of important differences:

differences in the standard are:

Voltage rating: 600/1000v
Range: 1.0mm to 6.0mm, 2c 3c & 4c plus earth

The other principle criteria called up in IS 273 are basically the same as used in BS8436.

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