Twin and Earth

[JohnW2]

Thanks, that clears it up now. So the only reason, from my understanding, is that a 20amp re-wire able fuse would hold an over current condition longer than an MCB. MCBs start their trip curve around 125%, correct?

Sort-of. An MCB is required to allow 113% of rated current to flow indefinitely without tripping, and must trip within 1 hour with 145% of rated current. I'm not sure of the 'indefinite time' figure for a re-wireable fuse, but the current needed to blow it within an hour is about 200%. 145/200 = 0.725, hence that 'correction factor'.

BTW, this only applies to re-wireable semi-enclosed fuses (like our BS3036). With ('enclosed') cartridge fuses one use sue the same figures as for MCBs.

Kind Regards, John

 

[mbrooke]

You may be correct about installation methods. There is no heading to the table.

It is French and from 1981 so ...

Ok, my mistake, I did not know it was that old.

The values for MCB seem to change insulation method, but I have been told they are "good rules of thumb"

I have to ask, with Twin and earth, what is the temperature rating of the individual PVC insulation on the conductors? Does it vary or a standard is in place?

 

[mbrooke]

Thanks, that clears it up now. So the only reason, from my understanding, is that a 20amp re-wire able fuse would hold an over current condition longer than an MCB. MCBs start their trip curve around 125%, correct?

Sort-of. An MCB is required to allow 113% of rated current to flow indefinitely without tripping, and must trip within 1 hour with 145% of rated current. I'm not sure of the 'indefinite time' figure for a re-wireable fuse, but the current needed to blow it within an hour is about 200%. 145/200 = 0.725, hence that 'correction factor'.

BTW, this only applies to re-wireable semi-enclosed fuses (like our BS3036). With ('enclosed') cartridge fuses one use sue the same figures as for MCBs.

Kind Regards, John

Brilliant!

 

[JohnW2]

I have to ask, with Twin and earth, what is the temperature rating of the individual PVC insulation on the conductors? Does it vary or a standard is in place?

I can't answer that directly, but the BS7671 CCC figures for twin & earth we're talking about are designed to avoid the temperature of the conductors rising above 70°C. If one complies with BS7671, the insulation and sheathing should therefore never have to handle a temperature higher than that. For what it's worth, IIRC, the melting point of PVC is somewhere in the 160-180°C region.

Kind Regards, John

 

[mbrooke]

I have to ask, with Twin and earth, what is the temperature rating of the individual PVC insulation on the conductors? Does it vary or a standard is in place?

I can't answer that directly, but the BS7671 CCC figures for twin & earth we're talking about are designed to avoid the temperature of the conductors rising above 70°C. If one complies with BS7671, the insulation and sheathing should therefore never have to handle a temperature higher than that. For what it's worth, IIRC, the melting point of PVC is somewhere in the 160-180°C region.

Kind Regards, John

70*c for the conductors, at least all adjustments put together will prevent the temperature going any higher. got it Earth fault loop impedance calcs assume 70*C Live and PE?

As for the terminations, must they also be rated 70*C? Or are they typically lower?

 

[JohnW2]

70*c for the conductors, at least all adjustments put together will prevent the temperature going any higher. got it Earth fault loop impedance calcs assume 70*C Live and PE?

Strictly speaking, they should.

As for the terminations, must they also be rated 70*C? Or are they typically lower?

I think one has to assume that anything in thermal contact with the conductors could rise in temperature to the maximum permitted temperature of the conductors.

Kind Regards, John

 

[mbrooke]

70*c for the conductors, at least all adjustments put together will prevent the temperature going any higher. got it Earth fault loop impedance calcs assume 70*C Live and PE?

Strictly speaking, they should.

As for the terminations, must they also be rated 70*C? Or are they typically lower?

I think one has to assume that anything in thermal contact with the conductors could rise in temperature to the maximum permitted temperature of the conductors.

Kind Regards, John

But what are the connectors and MCB terminals rated for themselves? Or they play no role in wire selection?

 

[RF Lighting]

Standard switchgear *should* be rated for 70°C connections, which is why CCC ratings for XLPE cables can't normally be used, as they are expecting 90°C terminations to be availible.

In the real world however, some cheaper switchgear will fail if exposed to prolonged heating of 70°C, and cable life expectancy may be reduced. Also it may not be desirable to have cables running at this temperature, especially in a domestic environment.

 

[mbrooke]

Standard switchgear *should* be rated for 70°C connections, which is why CCC ratings for XLPE cables can't normally be used, as they are expecting 90°C terminations to be availible.

In the real world however, some cheaper switchgear will fail at 70°C, and cable life spans may be reduced. Also it may not be desirable to have cables running at this temperature, especially in a domestic environment.

Is it assumed that a 60*C conductor temperature for example will produce a 60*c temperature at the terminals? Or they run hotter?

 

[RF Lighting]

It should do, as the terminals will be rated for the current they are passing so will not generate any additional heat, unless they are worn or damaged, but that is a problem regardless of the conductor temperature. The only difference is that a cold conductor may pull enough heat from a damaged terminal to prevent it failing.

 

[Adam_151]

I came across a 80A SPN switchfuse the other day, I wont say what brand, but its a certain national wholesalers version of a Wylex CM160. This was rated at 80A rather than the 60A for the wylex version. A label on the front however stated "for 80A rating, use 25mm cables". I can only assume that in order not to overheat at 80A, the switch required 25mm of copper to act as a heatsink

 

[JohnW2]

I came across a 80A SPN switchfuse the other day, I wont say what brand, but its a certain national wholesalers version of a Wylex CM160. This was rated at 80A rather than the 60A for the wylex version. A label on the front however stated "for 80A rating, use 25mm cables". I can only assume that in order not to overheat at 80A, the switch required 25mm of copper to act as a heatsink

Maybe. However, even if that is the case, it's probably rather different from what we are talking about (terminations) - since, as RF said, proper terminations should not generate significant heat. However, as eric is always pointing out, by definition, fuses get hot, potentially very hot when carrying currents close to (or above!) their In - so, IF the call for 25mm² for 80A use were related to 'heatsinking', it would presumably be primarily related to heat generated by the fuse, not the terminations.

However, I am a bit uncertain about your speculation, since specifying 25mm² cable 'as a heatsink' would be pretty meaningless without knowledge of the length of the cable - a few inches of 25mm² would not constitute much of a heatsink.

Kind Regards, John