FCU on storage heaters

[alamaison]

Been meaning to ask this here for a while: why would storage heaters be con= nected via a 13A FCU?

All round the house. Each storage heater is on it's own 2.5mm2 radial back= to 15A fuses in the Economy 7 CU but the radial connects to the heater fle= x via a 13 switched FCU. While some of the heaters are 2.5kW, and so shoul= d technically be ok in a 13A FCU, others are 3.4kW! It works but I'm not s= ure why those fuses don't blow.

Am I missing something? Is there any reason (even something back-in-the-da= y) why these heaters are on FCUs rather than 20A switches? If it makes a d= ifference, the house and its electrics are from the 50s.

Alex

[Owain]

onnected via a 13A FCU?

back to 15A fuses in the Economy 7 CU but the radial connects to the heater= flex via a 13 switched FCU. =A0While some of the heaters are 2.5kW, and so= should technically be ok in a 13A FCU, others are 3.4kW! =A0It works but I= 'm not sure why those fuses don't blow.

e-day) why these heaters are on FCUs rather than 20A switches? =A0If it mak= es a difference, the house and its electrics are from the 50s.

Lack of clue, or what was on the van that day.

Owain

[John Rumm]

In some cases they may be required to offer adequate fault protection to the flex between the connection and the heater when they are powered from larger circuits such as 20A or 32A ones. However in many it will simply be that FCUs are what were to hand at the time of fitting - switched flex outlets with neon and no fuse are less easy to find.

3.4kW is only 14A or so. If you look at the typical response of a typical BS1362 13A fuse like that used in plugs and FCUs:

will note that loads up to just over 20A will typically not blow the fuse.

In this case it sounds more like pragmatism in the circumstances rather than any technical reason.

[harryagain]

All round the house. Each storage heater is on it's own 2.5mm2 radial back to 15A fuses in the Economy 7 CU but the radial connects to the heater flex via a 13 switched FCU. While some of the heaters are 2.5kW, and so should technically be ok in a 13A FCU, others are 3.4kW! It works but I'm not sure why those fuses don't blow.

Am I missing something? Is there any reason (even something back-in-the-day) why these heaters are on FCUs rather than 20A switches? If it makes a difference, the house and its electrics are from the 50s.

Alex

The reason they are on radials is that there is no benefit of having a ring main as there is no diversity, (ie the heaters are likely to be all turned on at once.

The FCUs are used for many different purposes, storage heaters are just one. The fuse in them enables closer protection of whatever is connected to them. If the right sized protection is at the consumer unit, there is no reason why a simple local switch could not be used.

FCU with a fuse can also be used in a ring circuit with a 32a fuse back at the consumer unit for example. Though not for storage heaters in this example)

[Dave Plowman (News)]

Probably got them cheaper than non fused switched cord outlets.

[Andrew Gabriel]

The fuse will run hotter than designed, by around 25% more power dissipation than the 1W designed. This may cause more rapid deterioration of fuse holder contacts, even more heating, and earlier failure of other parts of the FCU, including the terminal connections.

[alamaison]

This accurately describes exactly what's been happening. One FCU even cracked around the 'door' to the fuse.

Thanks for all the replies. What I think I'll do is gradually replace them with

20A neon switches as the rooms get redecorated.

Alex

[meow2222]

onnected via a 13A FCU?

ck to 15A fuses in the Economy 7 CU but the radial connects to the heater f= lex via a 13 switched FCU. While some of the heaters are 2.5kW, and so sho= uld technically be ok in a 13A FCU, others are 3.4kW! It works but I'm not= sure why those fuses don't blow.

day) why these heaters are on FCUs rather than 20A switches? If it makes a= difference, the house and its electrics are from the 50s.

A local fuse has a second advantage that at least some of the time, a fault= y heater will blow its 13A fuse, leaving the rest functioning. If you've go= t one 15A heater per 15A fusebox way then of course this doesn't apply, and= a 2nd fuse isn't useful.

15A is ok on 13A accessories if intermittent, but not continuous. I'd repla= ce any FCU that shows signs of stress, but if they've been ok for 50+ years= then they were presumably capable when fitted, albeit without the level of= margin one would want today.

NT