6.5.2 - Cooker circuits
A cooker is regarded as a piece of fixed
equipment unless it is a small table-mounted type fed from
a plug by a flexible cord. Such equipment must be under
the control of a local switch, usually in the form of a
cooker control unit. This switch may control two cookers,
provided both are within 2 m of it. In many cases this control
unit incorporates a socket outlet, although often such a
socket is not in the safest position for use to supply portable
appliances, whose flexible cords may be burned by the hotplates.
It is often considered safer to control the cooker with
a switch and to provide a separate socket circuit. The protective
device is often the most highly rated in a') installation,
particularly in a domestic situation, so there is a need
to ensure that diversity has been properly calculated (see
Table {6.2}).
The diversity applicable to the current
demand for a cooker is shown in
{Table 6.2} as 10 A plus 30% of the remainder of the
total connected load, plus 5 A if the control unit includes
a socket outlet. A little thought will show that whilst
this calculation will give satisfactory results under most
circumstances, there is a danger of triggering the protective
device under some circumstances. For example, at Christmas
it is quite likely that both ovens, all four hotplates and
a 3 kW kettle could he simultaneously connected. Just imagine
the chaos which a blown fuse would cause! This alone is
a very good reason for being generous with cable and protective
ratings.
Example 6.2
A 240 V domestic cooker has the following connected
loads:
| top oven |
1.5 kW |
| main oven |
2.5 kW |
| grill |
2.0kw |
| four hotplates |
2.0 kW each |
The cooker control unit includes
a 13 A socket outlet. Calculate a suitable rating for the
protective device.
The total cooker load is 1.5 + 2.5 + 2.0
+ (4 x 2.0) kW = 14 kW
| Total current |
= P =
|
14000 A
|
= 58.3A |
| |
U
|
240==
|
|
| |
|
|
|
| |
|
|
|
The demand is is made up of:
| the first 10 A |
|
|
= 10.0 A |
| + 30% of remainder |
= 30 x (58.3 - 10) =
|
30 x 48.3
|
= 14.5 A |
| |
100
|
100
|
|
| + allowance for socket outlet |
|
|
= 5.O A |
| total
= |
|
|
29.5 A |
A 30 A protective
device is likely to be chosen. The cable rating will depend
on correction factors (see
{Chapter 5}).
|