Difference between revisions of "CU"

Revision as of 00:02, 10 March 2009

Work in progress

The main articles dealing with Consumer units, aka fuseboxes, are:

The CU, Consumer Unit or Fusebox provides fault current protection for all a house's electical circuits. It usually contains a switch to control all the circuits, and many also feature leakage current protection with RCD or RCBOs.

Split Load CU

A split load CU has 2 halves (sometimes more) with a separate power switch for each half. So some of your wiring can be turned off while the rest stays on. So whatever may happen with your electrical system, you don't need to turn it all off. This enables you to have power in the event of a fault, and to have power while working on the installation itself. (There are exceptions of course, such as lightning strike or a major fire.)

Split load CUs are common today (especially with wiring compliant with the 17th edition of the wiring regs), and can be recommended in nearly all cases.

Upgrades

Its often assumed that, being newer, MCBs provide better safety than fuses, but the level of protection both provide is similar, and both have pros and cons.

The main ways in which CUs can be upgraded are:

Fitting a split load unit
Elimination of touchable live parts, as found
- on many CUs when fuses are pulled,
- always live parts are common on pre-war fuseboxes, though these aren't often seen now
Incorporation of leakage protection
- with RCD(s)
- with circuit discrimination using RCBOs
Increasing the number of circuits rather than extending existing ones
Adding provision for a shower, cooker, or other high current load
Providing a separate feed for an outbuilding
Replacement of 3kA retrofit MCBs with 6kA MCBs.
Replacement of an inadequate, damaged or faulty CU
Making an old installation look modern

Upgrading CUs on TT installs

A little care is required over earth leakage protection when replacing a CU on a TT install, as a CU setup not specifically suited to TT use is likely to leave the system dangerous.

CUs on TT systems should be insulated (ie plastic), not metal.
A CU without full RCD cover should never be used where there is no preceding whole house RCD or ELCB.
Where RCD is housed in the CU, a single RCD covering the whole house is no longer compliant, and can cause loss of power. A split CU with 2x RCDs is usually a good solution.

MCBs & Fuses

Ratings

Lighting: Usually 6A MCB or 5A fuse. 10A circuits are sometimes used
Immersion heater: 15/16A
Socket circuits, ring: 30A fuse, 32A MCB
Socket circuits, radial: 20A
Cooker, shower: Usually 45A or more
Small loads: 2.5A, 5A, 6A.

For information on type B, C and D MCBs, see MCB

Level of protection

In an ideal world all circuits would have suitable overcurrent protection as well as fault current protection. In practice there are a percentage of existing installs that fail to meet this requirement.

Live parts

A lot of fuseboxes/CUs are in use that expose the user to live parts if fuses are pulled with the power on. Always turn power off before pulling a fuse.

Working on the CU

The switch in the CU is the main power switch for most houses. Thus exposed wiring will be live when the CU cover is removed, even with the switch off. Removing the main supply fuse before the meter solves this, but is illlegal, but its common practice nonetheless. Power companies have so far not objected to the practice, for an assortment of legal, safety and financial reasons.

Trips

Tripping or repeated fuse blowing are normally caused by a fault outside of the CU itself.

RCD trips are usually caused by a faulty appliance or a less than ideal installation design. See RCD
MCB trips are most commonly caused by blowing filament bulbs. Faulty appliances often also cause this.
RCBO trips may be due to either of the causes above, since an RCBO combines the functions of both MCB and RCD.

There are of course other possible causes of trips and blown fuses, the above is just a pointer to the most likely.

Brands

Ashley

Contactum

Hager

Wider than usual range of MCBs and RCBOs, including double pole.
Snappable busbar with good quality slip on sleeve makes live working safer

and allows custom split load configurations to be made

MCBs and RCBOs comptable across the domestic plastic and inductrial metal

enclosures.

Double pole (SPSN) and single pole devices may be mixed freely on a

standard single phase busbar (insulated busbar way on the neutral side and flying lead for the neutral). Double pole devices are general double width.

MEM

Expensive, but RCD "pods" are available for some of the MCBs allowing many

custom RCBOs to be constructed.

Probably the only manufacturer listing a true double pole busbar carrying

both phase and neutral on a split comb arrangement, avoiding the need for "flying neutrals".

MK

Square-D

Innovative enclosed and hidden busbar which allows devices to be clipped

in without removing the busbar from all devices.

Wickes

Wylex

Cheap & basic Wylex fuseboxes are very common in existing installs
Fuses expose live parts if pulled with power on
Overheating occasionally happens on high current circuit fuses (greater than 30A)

@@ Line 1: / Line 1: @@
 '''Work in progress'''
+The main articles dealing with Consumer units, aka fuseboxes, are:
+* [[Changing a consumer unit]]
+* [[Consumer Units]]
+* [[House Wiring for Beginners]]
+* [[Rewiring Tips]]
 The '''CU''', '''Consumer Unit''' or '''Fusebox''' provides fault current protection for all a house's electical circuits. It usually contains a switch to control all the circuits, and many also feature leakage current protection with [[RCD]] or [[RCD|RCBO]]s.