fused spurs / double Pole Switches
- Thread starter ColinHackett
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So would I plugwash, but that's what the standard says. I left out the bit about portable socket-outlets, for which the terminal temperature rise is 52 K. Presumably that's because the connections are to a single 1.25 mm2 3-core flex rather than a ring.
It's certainly not very well worded - but it's also true that there is no test specified in BS1363 which would be able to confirm that double sockets are safe in use beyond 20A total.
Kind Regards, John
Whoops! 
I should have read further. For the heat rise test, a multiple socket outlet having more than two sets of socket contacts is connected to an incoming 4mm2 T & E cable, with no outgoing cable. A single socket-outlet is connected to an incoming and outgoing 2.5mm2 T & E, but a twin socket-outlet is connected to an incoming cable "as described above" i.e. 2.5mm2 but no outgoing cable.
Curiouser and curiouser! They seem to have assumed that twin outlets will only be used as spurs or at the end of radials!
I should have read further. For the heat rise test, a multiple socket outlet having more than two sets of socket contacts is connected to an incoming 4mm2 T & E cable, with no outgoing cable. A single socket-outlet is connected to an incoming and outgoing 2.5mm2 T & E, but a twin socket-outlet is connected to an incoming cable "as described above" i.e. 2.5mm2 but no outgoing cable.
Curiouser and curiouser! They seem to have assumed that twin outlets will only be used as spurs or at the end of radials!
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So would I - but maybe we're all underestimating the extent of heat loss from terminals via the connected conductors?
Kind Regards, John
That will be substantial John - and remember that the use of the cable is limited to 70 deg C, so we shouldn't be surprised.
Indeed. Does the test specify an ambient temperature? If ambient was >23C°, then a 47° rise in terminal temp would presumably take the temp of the conductor in the cable to above 70°C ! Indeed, maybe that's where the 47° figure comes from!
Kind Regards, John
Nothing so simple John! Tests are performed at an ambient temperature of 20 +- 5 deg C. The limits are specified to take account of a maximum uncertainty of measurement of +- 2 deg C at a confidence level of not less than 95%.
Apologies to all for being too lazy to format these expressions correctly!
Apologies to all for being too lazy to format these expressions correctly!
That's slightly ambiguous. Does it mean that the measured ambient temperature has to be 20 ± 3 °C, so as to keep it within 20 ± 5 °C after worst acceptable measurement error is taken into account, or does it mean that the measured ambient temperature can be 20 ± 5 °C, so that the actual temp (in the face of worse acceptable measurement error) is within 20 ± 7 °C? Even the former of those could have terminal temp (hence cable conductor temp) rising to 72 °C with a 47° rise!
Kind Regards, John
Sorry, I should have entered 2 separate paragraphs. The tests are performed at 20+-5 deg C.
The +-2 uncertainty allowance is included in the table of allowable temp rise.
Presumably the ambient is easy to measure precisely, but the temperature of the terminals and external surface are measured using external thermocouples glued on so the uncertainty will be greater. Does that make sense? You're better at statistics than I am!
The +-2 uncertainty allowance is included in the table of allowable temp rise.
Presumably the ambient is easy to measure precisely, but the temperature of the terminals and external surface are measured using external thermocouples glued on so the uncertainty will be greater. Does that make sense? You're better at statistics than I am!
OK, but still a little ambiguous
Do you mean that, say, the 47° maximum permissible measured temp rise is 47°, such that the true temp rise (with ±2° permissible error) might be as high as 49° (or that the maximum measured rise is 45°, so that the true rise is not above 47°) ?That's really physics or engineering, rather than statistics - but I would imagine that you're probably right in suggesting that there will be less accuracy (i.e. more uncertainty) in the measurement of terminal temp than ambient temp!
Kind Regards, John
John, I'm only quoting what's in the standard. I didn't write it so have no more idea what was meant than any other reader. I thought it might make more sense to you than to me.
In view of John's recent calculations of the resistance of heating elements, what would be the actual current of two 3kW (at 20°) appliances plugged in to a double socket after the initial few minutes?
Tell me how hot the heating elements will be 'after the initial few minutes' and I'll do the approximate sums for you
The other crucial question is, of course, whether you are right in saying that a '3kW appliance' is 3kW 'when cold' - or whether the 3kW actually relates to 'after the first few minutes'. After all, with filament lamps, we quote the wattage when hot, not cold (the latter of which would be extremely high, for at least a few milliseconds).Kind Regards, John
I realise that but, although I may have a few minor skills, the ability to read the minds of those who write unclear and/or ambiguous statements in Standards is not one of them
However, as I've said, unless the figures are just a coincidence, it does sound as if the permitted temp rise of terminals (given the ambient specified for test) may be at least partially related to the need to avoid the conductors of attached cables rising above 70°C. It would, after all, be a bit daft for BS1363 to permit terminals (hence cable conductors) to rise to a temperature above the maximum permitted for the vast majority of cables connected to such terminals!
Kind Regards, John
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