1.5 mm T&E on a B32 MCB

[Pete999]

replacing like for like, just thought I'd do a quick Ze, IR and check the RCD and noticed the the 1.5 in the B32.

Thanks for the reply

 

[Ian1981]

Hager let through energy on a 32 amp mcb type B is 29kA so in this case the 1mm cpc for the 1.5mm twin and earth cable would be undersized and inadequate for fault protection.
I2t would be greater than the K2S2 of the cable.

 

[GBDamo]

1.5 is the minimum for a power circuit, if the only thing connected is a fused spur then it can't pull no more than 13A. If the installation method permits adequate CCC and the CSA is adeqate for fault current/thermal constraints under fault conditions then technically there is nothing wrong with it.

That is all true but very close the knuckle.
VD works out at 2.02% so ok, is Ref method A (14amps) again ok but, the point of the FCU is to protect the cable from the FCU to the appliance and the MCB to protect the cable from the MCB to the FCU/accessory. In the event of eg. a line - neutral fault the 1.5mm would overload before the 32Amp MCB would trip.

 

[GBDamo]

Hager let through energy on a 32 amp mcb type B is 29kA so in this case the 1mm cpc for the 1.5mm twin and earth cable would be undersized and inadequate for fault protection.
I2t would be greater than the K2S2 of the cable.

Nope, that's going to take me much more research to figure out.

 

[Ian1981]

Nope, that's going to take me much more research to figure out.

For faults on current limiting devices such as bs en 60898 devices under 0.1 of a second then the I2t let through energy needs to be sought for compliance that the K2S2 energy withstand of the cable under fault conditions and thermal constraints is greater than the let through energy quoted by the manufacturer.
In the example of a hager mcb ,technical info I have then the 1.5mm is inadequate to be connected to a 32amp type b circuit breaker.
That is if the tripping time exceeds 0.1 of a second.
1.5 would be the minimum so the live conductor is satisfactory but the cpc being 1.0mm is not.

 

[Toneyz]

Or change it for a 6Amp cb

If it is an up to date of the shelf MCB for the extra cost to the customer this can be the safest option or disconnect with permission.

 

[Risteard]

VD works out at 2.02% so ok

In the absence of a connected load there cannot be a voltage drop.

 

[GBDamo]

In the absence of a connected load there cannot be a voltage drop.

Assumed 13amps worst case scenario for the FCU.

 

[Bobster]

Hager let through energy on a 32 amp mcb type B is 29kA so in this case the 1mm cpc for the 1.5mm twin and earth cable would be undersized and inadequate for fault protection.
I2t would be greater than the K2S2 of the cable.

I'd like to see a domestic installation, after fault limiting by the DNO fuse, that has anywhere near 29kA I2t. I'd dare say you'd be hard pressed to find anywhere near 10kA

 

[happysteve]

Hager let through energy on a 32 amp mcb type B is 29kA so in this case the 1mm cpc for the 1.5mm twin and earth cable would be undersized and inadequate for fault protection.
I2t would be greater than the K2S2 of the cable.

To get an energy let-through of 29kA^2 s on a Hager B32 you'd need a fault current of about 6kA, according to this graph:

(Source: Hager General Catalogue 2017) (p94 of the PDF)

For this particular device, 1mm cpc would be ok for fault currents up to about 3kA (max energy let-through 13225A^2 s (=k^2 = 115^2).

More generally, table B7 in the On-Site Guide gives minimum protective conductor size based on the energy limiting class 3 device (e.g. BS EN 60898/61009), this is the maximum energy let-through permitted by this standard. Individual manufacturers have a lower I^2 t than this, but the table gives a nice "worst case" limit.

As an aside, if you've ever wondered what the "3" in the square is, under (usually) 6000 or 10000 in a rectangle (which is Icn) on the side of a circuit breaker, then it denotes the energy limiting class of the device. I have never seen one that isn't a class 3. The alternative seems to be "class 1" for which there are no limits specified.