Spur from Ring Final Circuit

[Gigsy]

Get used to it. Every trade and profession has its own specific lingo. Just ask or Google if you don't know.

Here's a few for you to look up before the next lesson. Hint: there's a list of abbreviations in the BYB.
DNO, ELV, SELV, RCBO, RCCB, FCU, BC, CPC, DB, GLS, IR, MFT, PIR, RFC, SES, SWA, UKPN

I'll have a stab at them without looking in the BYB.

DNO = Distribution Network something
ELV = Extra low voltage
SELV = Safety extra low voltage
RCBO = Residual current breaker with overcurrent
RCCB = dont know but same as RCD
FCU = Fused connection unit
BC =
CPC = Circuit procetive conductor
BD = Distribution board
GLS =
IR = Infra red
MFT =
PIR = don't know motion sensor thing
RFC =
SES =
SWA = don't know armoured cable
UKPN

 

[Gigsy]

Because the junction box you have shown will not take all the cables (max of 4) you show 5

Good observation, however I believe you can push two 2.5mm2 cables into the push terminal, it says so in the lid of the connection box

 

[Gigsy]

Because the junction box you have shown will not take all the cables (max of 4) you show 5

You can actually have a max of 8 cables

 

[Gigsy]

You are correct we can't know what loads will be applied to what sockets but for sure feeding that many spurs into one point on the ring makes heavy loads on those spurs more likely to unbalance the ring. Therefore the design isn't suitable for ensuring a normal distribution of usage would create a balanced load.

That's my view on looking at the diagram and my interpretation of the regs based on it.

Thank you for your view, it is a view I share, but if you had an appliance that drew allot of current on the last socket, would that design then be acceptable?

 

[Pat H]

What last socket? You have a ring so no end as such and spurs? But in a normal ring (RFC Ring Final Circuit) any heavy load anywhere on the ring has the benefit of its supply coming both directions down the ring.
Tapping a lot of spurs from one point of the ring is concentrating loading on the ring to that point. A single double socket could draw 26A max from that point on the ring. Each spur can take up to 26A so with 2 you have 52A 3 65A 4 104A
The ring will be protected by an overload device but we don't know what that is and what its characteristics will be but its possible we could be drawing a lot of current via that one junction box.

 

[Gigsy]

What last socket? You have a ring so no end as such and spurs? But in a normal ring (RFC Ring Final Circuit) any heavy load anywhere on the ring has the benefit of its supply coming both directions down the ring.
Tapping a lot of spurs from one point of the ring is concentrating loading on the ring to that point. A single double socket could draw 26A max from that point on the ring. Each spur can take up to 26A so with 2 you have 52A 3 65A 4 104A
The ring will be protected by an overload device but we don't know what that is and what its characteristics will be but its possible we could be drawing a lot of current via that one junction box.

The junction box is rated at 30A, you cant pull more than 32A through the circuit for any length of time anyway. Each double socket can pull 26A and you can have an unlimited amount of sockets so adding up the limits that can be pulled is not really relevant when considering the design of a RFC. The socket marked red in the picture is the one I was referring to as the last socket. Each spur can pull 26A, 13A each single socket and 2.5mm2 cable in certain conditions is rated at 26A, so there is no problem there. The JB pulls current from the left cable and the right cable a total of 52A, can be supplied.

You could be right I guess, is it only that the junction box is the weak point? What if you have a junction box rated at a higher ampage?

 

[HandySparks]

I'll have a stab at them without looking in the BYB.

DNO = Distribution Network something
...
SWA = don't know armoured cable
UKPN

Not bad. But worth looking them up. It's easy to guess at something that's nearly right, but not quite.

 

[Gigsy]

Forgot the picture!!

 

[Gigsy]

Not bad. But worth looking them up. It's easy to guess at something that's nearly right, but not quite.

Will do, thank you

 

[Barry White]

You can actually have a max of 8 cables

Not acording to the manufacturer!

 

[Gigsy]

View attachment 34351

View attachment 34352

Not acording to the manufacturer!

If you look in the lid it says you can put 2 wires in each hole. I know it says 2 1mm2 wires can go into a hole when you push the button in to open the hole more, not 100% sure if you can put two 2.5mm wires in one hole. The cable grips grip 2 cables by default, you have to turn them upside down to grip one cable.

 

[Gigsy]

View attachment 34351

View attachment 34352

Not acording to the manufacturer!

Does anyone have j803, can you confirm in the lid it says you can put 2x 1mm2 wires in one hole when you push in the button to make the hole bigger?

 

[Pat H]

I'd lookup the actual current over protection devices will trip at. You can take more than 32 A from a 32A protection device and that varies on the device.
BS7671 does state that there maybe ways to achieve the requirements and if you think you can argue thst case then alls fine.

 

[Gigsy]

I'd lookup the actual current over protection devices will trip at. You can take more than 32 A from a 32A protection device and that varies on the device.
BS7671 does state that there maybe ways to achieve the requirements and if you think you can argue thst case then alls fine.

You can take more than 32A from a 32A MCB for a limited amount of time, until the bimetallic strip in the device gets hot enough and bends far enough to trip it. It will eventually trip if you pull more than 32A on all BS rated devices.

I believe the chart you may be thinking of is the one that shows how much current is needed in a short circuit fault situation to trip the device in 0.4 seconds, That will activate a solenoid and instantly trip the MCB

 

[Gigsy]

View attachment 34351

View attachment 34352

Not acording to the manufacturer!

Says you can inside the box