I believe 'k' should also be dimensionless, if it is truly called a 'constant'.
If BS7671 uses this wording then all I can say it is misleading. Especially if it creates a presumption that the cable can accept a temperature increase of 160C, on top of the 70C max design temperature.
I have the 17th ed regs somewhere, which section is this in?
This article suggests the max working temperature ought to be 60C. And 'Melting temperatures range from 212°F to 500°F (100°C to 260°C) depending upon manufacture additives to the PVC'.
Not a village hall in this case but the back of a Television Outside Broadcast Tender (lorry carrying the cable, camera, vision and sound equipment). The riggers sound and vision crew tend to check what supply the tender was on (13 A fused or 16 A unfused) before using kettle and urn together. Camera's just use the kettle without checking, then don't understand why it's taking a long time to boil and the lights have gone out...
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Don't like the tight bend over the ends.
Much better to learn how to coil a cable properly and use a velcro strap to hold the coil together.
Agreed, though for extensions that's not some much of a problem if they are coiled properly. I replace the (sometimes) crappy PVC flex on power tools with rubber sheathed for the better handling and flexibilty.
k is a "factor" rather than a constant. I varies with the type of cable and the assumptions made about start temperature etc.
The adiabatic article you linked to previously (good site BTW!) shows a method for deriving k factors based on the temperature difference, heat capacity and density of the conductor material, and the resistivity of the material.
There is another quite nice explanation here:
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For practical purposes, values normally taken from table 54.3 of BS7671 (i.e. assuming an initial temperature of 70 deg C, and copper cable)
It doesn't create that presumption - I think you are reading rather too much into it. Note also that 160 deg C is the maximum temperature, not the delta. So it can accept an increase of 90 deg on top of the 70.
The limiting temperatures are quoted in Table 43.1
(note you can get slightly less pessimistic answers from the "adiabatic" equation if you use k factors that are derived allowing for some non-adiabatic heat transfer. There is a whole BS on that snappily titled "BS 7454 Calulation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects")
543.1.3 (yellow book)
Most standards seem to take the maximum conductor temperature as either
70 or 75 for PVC insulated cables AFAIAA. Note that the conductor temperature will be somewhat higher than the overall working temperature since the insulation will normally be losing heat to its surroundings and the outer temperature will be lower than that adjacent to the conductor.
I see. I know of a 2.5 T&E ring circuit that frequently runs 45A and more, since it powers a kitchen, lounge, corridors and another much used room. an d and naturally currents aren't balanced across the 2 runs of T&E back to t he CU. How is that not operating beyond its ratings?
And yet there is no sign of damage to the cable. I know because I've inspec ted it and recommended work.
Well that may become more of an issue in modern houses that have more insulation. Off the top of my head a 2.5mm cable dropping down an insulated plasterboard wall with the cable touching the plasterboard has a CCC of 21A.
A short blast of 30A down this cable (kettle boils just as the washer decided to heat up a bit of water and the toaster is also on etc) on an unbalanced ring will do it no harm. Normally an attempt is made to iron out the unbalanced ring by putting high loads onto a separate radials or using two rings. That you know has always been the case, however IMHO with the more insulated houses this is far more important.
One example of overheated 1.5mmT&E I have seen was an old immersion supply that was was laid across old loft insulation. Later more insulation was added. In this case the MCB tripped and is a testament to the fire retardant abilities of that rock wool.
The 500 kW twinset generators are stable at 240 V despite basicly only idling when only loaded to 30 kW or so. You don't want to over or under volt several tens of million pounds outside broadcast truck...
Glad you've learned something. Another might be that it's not unusual to use 1.5mm MICC for a ring - because the insulation can stand a great deal more heat.
4) He did it perfectly and you made a helpful suggestion about a way he could make it just a tiny bit better, for which he thanked you, then made the tea and swept the floor?
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