How do fused spurs FCUs work.

mavezy · Jan 27, 2023

Hi, quick question. How do FCU's work? Do they just protect the load side and what I mean is, say you have a ring final with 2.5mm^2 protected by a 20a mcb at the consumer unit and you add a fused spur on that ring de-rated to 3a to supply a 1mm cable for lighting. Now say you start turning on kettles, toasters, microwaves on that ring that pull more than 3a, is that 3a in the fused unit going to pop? Or will it only pop if the load side on the FCU pulls more than 3a?

Just an electrical enthusiast who was watching a video on it and wondered how it actually works.

cheers

nicebutdim · Jan 27, 2023

Ring should be connected through the supply side, so only load side is protected by the fuse.

mavezy · Jan 27, 2023

Ah cool thanks for the quick reply!

brigadierbob · May 2, 2023

mavezy said:
Hi, quick question. How do FCU's work? Do they just protect the load side and what I mean is, say you have a ring final with 2.5mm^2 protected by a 20a mcb at the consumer unit and you add a fused spur on that ring de-rated to 3a to supply a 1mm cable for lighting. Now say you start turning on kettles, toasters, microwaves on that ring that pull more than 3a, is that 3a in the fused unit going to pop? Or will it only pop if the load side on the FCU pulls more than 3a?

Just an electrical enthusiast who was watching a video on it and wondered how it actually works.

cheers

Hi Mavezy, firstly a Final Ring Circuit should be protected by a 32A overload protective device, a BS 60898 Mcb, A BS1361 Fuse at the DB/Consumer Unit or a BS 1363 at the FCU. All of these forms of protection are installed for protection against overload and earth live short circuit. Usually nowadays we have additional circuit protection by RCDs, or Residual Current Detectors, for their proper name. These devices detect unusually high currents in connected devices, but not overload or short circuit faults. Basically if an FCU is pulling 35 amps,the 3A fuse will not trip, but the RCD at the Consumer unit will.

nicebutdim · May 2, 2023

brigadierbob said:
Hi Mavezy, firstly a Final Ring Circuit should be protected by a 32A overload protective device, a BS 60898 Mcb, A BS1361 Fuse at the DB/Consumer Unit or a BS 1363 at the FCU. All of these forms of protection are installed for protection against overload and earth live short circuit. Usually nowadays we have additional circuit protection by RCDs, or Residual Current Detectors, for their proper name. These devices detect unusually high currents in connected devices, but not overload or short circuit faults. Basically if an FCU is pulling 35 amps,the 3A fuse will not trip, but the RCD at the Consumer unit will.

Have to disagree with you here.

RCD or Residual Current Device measures current in line and neutral, tripping when imbalance between these approaches a set threshold.

An RCD does not incorporate any means of overload protection and will not trip as you describe.

mainline · May 2, 2023

brigadierbob said:
Hi Mavezy, firstly a Final Ring Circuit should be protected by a 32A overload protective device, a BS 60898 Mcb, A BS1361 Fuse at the DB/Consumer Unit or a BS 1363 at the FCU. All of these forms of protection are installed for protection against overload and earth live short circuit. Usually nowadays we have additional circuit protection by

brigadierbob said:
RCDs, or Residual Current Detectors, for their proper name.

And there is me thinking RCD stood for residual current device.

brigadierbob said:
These devices detect unusually high currents in connected devices, but not overload or short circuit faults.

That statement doesn't make sense

brigadierbob said:
Basically if an FCU is pulling 35 amps,the 3A fuse will not trip, but the RCD at the Consumer unit will.

Neither does this one.

James · May 2, 2023

an rcd will not detect unusually high current providing it is equal in the live and N it will not trip.
a 3A fuse will not survive 35A for long, trust me it will blow but I don’t have the time/current chart to hand so I can’t say how long it will survive but it will be measured in seconds, not minutes.
edit, just looked it up, less than 0.1 seconds

pc1966 · May 2, 2023

You can think of a FCU has a "hard-wired 13A plug" in a way. The load has the same type of fuse to protect the cable and load (selected to match, usually 3A for lights, etc, and 13A for a big load), but the supply side is basically the same as any other 13A socket has (typically 32A).

pc1966 · May 2, 2023

James said:
a 3A fuse will not survive 35A for long, trust me it will blow but I don’t have the time/current chart to hand

It is under 20ms

[ElectriciansForums.net] How do fused spurs FCUs work.

James · May 2, 2023

brigadierbob said:
Hi Mavezy, firstly a Final Ring Circuit should be protected by a 32A overload protective device, a BS 60898 Mcb, A BS1361 Fuse at the DB/Consumer Unit or a BS 1363 at the FCU. All of these forms of protection are installed for protection against overload and earth live short circuit. Usually nowadays we have additional circuit protection by RCDs, or Residual Current Detectors, for their proper name. These devices detect unusually high currents in connected devices, but not overload or short circuit faults. Basically if an FCU is pulling 35 amps,the 3A fuse will not trip, but the RCD at the Consumer unit will.

as a first post to the forum, it was a shame that even though you list yourself as highly qualified lecturer of the subject,
it was riddled with technical errors, why not try posting in the welcome forum nd having a chat about what sort of things you have been involved with?
we are a friendly group and not a bunch of trolls here
however if you post something that is wrong on A Technical point, it will be noticed and pointed out.