Combining potentially dangerous electrical devices with
watery environments is a phenomenon that is almost -but not
quite - unique to the UK and now these devices, more commonly
known as electric showers, are also becoming more and more
powerful. The implications of installing the higher kilowatt
electric showers are discussed by Roger Crabb, Marketing Communications
Manager of Caradon Showers.
These increasingly large kilowatt ratings -such as 10.8kW
versions -make it even more important that installers follow
manufacturers' instructions diligently and understand the
implications that the addition of the shower will have within
the overall electrical supply): It is, of course, also essential,
that all the rules and regulations governing such installations,
as set out in the 16th Edition of the lEE Wiring Regulations
-BS 7671- are stuck to. The following extracts from the 16th
Edition of the IEE Wiring Regulations BS_7671, together with
some pointers on best practice, are designed to highlight
various criteria that installers must take into account when
fitting electric showers.
It is necessary to determine if the existing electrical supply
to a building is suitable for the selected electric shower
unit and has sufficient spare capacity to allow the shower
unit to be installed. Regulation 313-01-01 requires the following
to be determined by calculation, measurement, enquiry or inspection.
(i) the nominal voltage -the standard voltage in the UK is
230 volts + 10% -6%, check rating plate of shower unit
(ii) the nature and frequency of current -normally alternating
current at 50Hz frequency
(iii) the prospective short-circuit current at origin of
the installation - this is to ensure the correct type of over
current device is selected (i.e. type of fuse or miniature
circuit breaker)
(iv) the earth fault loop impedance of that part of the system
external to the installation -to verify the earth arrangement
-is adequate for any earth fault currents
(v) the suitability for the requirements of the installation,
including the maximum demand -the normal capacity of the supply
to modern domestic premises is 1OOA -but maximum demand should
be reassessed.
(vi) the type and rating of the overcurrent protective device
acting at the origin of the installation -the rating of the
supply fuse in modern premises is normally 100A but older
housing stock can be as little as 60A- the fuse characteristic
against which the maximum demand value is compare
For more detailed information, installers should refer to
the 16th Edition of the IEE Wiring Regulations or contact
Mira for its own guide to Electric Shower Installations.
Points (i) and (ii) above are self explanatory however, Protective
device points (iii) and (iv) Regulation 314-02-01 to 314-
02-04 deals with the installation circuit arrangement. A shower
must be supplied from a dedicated radial circuit protected
by its own overcurrent protective device. Point (iii) -The
prospective short circuit current (PSCC) value can be obtained
from the Electrical Supply Company.
Protection devices
The type of protective device selected must have a breaking
capacity of not less than the PSCC at their point of installation.
As a guide mcbs to BSEN 60898 and fuse to BS 3036 are coded
to indicate the breaking capacity:
- mcb M6 -6000A 6kA fuse S1-1kA
- mcb M9 -6000A 9kA fuse S2 -2kA
In an existing installation the type of overcurrent protective
device will have been selected for the existing circuits,
the above may be used as a check by the electrician.
Regulation 601-04-01. The characteristics of the protective
device and the earthing arrangements shall be such, that in
the event of a fault to earth, disconnection occurs within
0.4 seconds.
The disconnection time of 0.4 seconds must be assured by
compliance with chapter 41, section 413-02 of the regulations.
It is a function of the type of wiring system supplied by
the electrical supply company -the value of the earth loop
impedance and the type of overload protective device selected.
However, if the requirements cannot be fulfilled it is recommended
under regulation 413-02-04 (ii) protection shall be provided
by means of a residual current device (rcd). The rcd should
have a trip rating of 30mA, a continuous current rating of
at least equal to that of the shower unit and be used in conjunction
with the overload protective device. The introduction of the
rcd for shock protection and the circuit impedance is limited
only by the need to avoid excessive volt drop on the connecting
cables. Maximum demand -the rating of the shower unit must
be taken in full -no diversity is allowed.
The installer should also check the effect of the shower
unit on the overall maximum demand of the dwelling. Refer
to Appendix J of the 16th Edition of the IEE Wiring Regulation
Guidance Note No.1 for the diversity factors allowable. The
maximum demand is the total of all circuit loadings after
the diversity factors are applied. If the assessment indicates
there is insufficient capacity in the existing supply, it
will be necessary to request an increase from the electrical
supply company.
This is likely to be necessary if more than one shower unit
is being installed unless a satisfactory method of ensuring
only one unit can be operated at a time such as electrically
or manually interlocked contactors which isolate the second
unit when the first is in operation.
Sizing of cables
Details the method of sizing the cables for connecting the
shower can be found in Section 6 of Guidance Note 1 -and takes
into consideration current demand capacity of conductors,
ambient temperature, bunching, volt drop and length of cable
run etc.
Locations containing a bath tub or shower tray/enclosure
Regulation 601-04-02 supple- mentary bonding shall be provided
between simultaneously accessible exposed conductive parts
of equipment and simultaneously accessible extraneous conductive
parts of equipment. Extraneous conductive parts may include
the following:
- Metal baths
- Taps
- Exposed metal pipes for hot and cold water
- Exposed metal wastes pipes
- Centrally heated radiators and towel rails
- Exposed central heating pipework
- Devices for Isolation or Switching
A circuit supplying a shower must have an isolating switch
of adequate rating near the shower that complies With Regulation
6014841 in the method of operation. An accepted method is
to use a ceiling-mounted pull cord switch having a 3mm contact
gap and reliable indication of when the separation distance
has been achieved. |
