7.11.2 - Electrode
boilers and water heaters
An electrode heater or boiler is a device
which heats the water contained, or raises steam.
Two or three electrodes are immersed in
the water and a single- or three-phase supply is connected
to them. There is no element, the water being heated by
the current which flows through it between the electrodes.
Electrode heaters and boilers must be used on a.c. supplies
only, or electrolysis will occur, breaking down the water
into its components of hydrogen and oxygen. The different
requirements for single-phase and three-phase boilers are
shown below.
A - Three-phase heaters fed from a low
voltage supply {Fig 7.13}
Fig 7.13 Three-phase
electrode heater fed from a low voltage supply
The requirements are:
1. - a controlling circuit breaker
which opens all three phase conductors and is provided with
protective overloads in each line,
2. - the shell of the heater bonded
to the sheath and/or armour of the supply cable with a conductor
of cross-sectional area at least equal to that of each phase
conductor,
3. - the shell of the heater bonded
to the neutral by a conductor of cross-sectional area at
least equal to that of each phase conductor.
B - Three phase electrode heaters fed
from a supply exceeding low voltage {Fig 7.14}
Fig 7.14 - Three-phase
electrode boiler fed from a supply exceeding low voltage
The requirements are:
1. - a controlling circuit breaker
filled with residual current tripping, set to operate when
the residual current is sustained arid exceeds 10% of the
rated supply current. Sometimes this arrangement will result
in frequent tripping of the circuit breaker, in which case
the tripping current may be reset to 15% of the rated current
arid/or a time delay device may be fitted to prevent tripping
due to short duration transients. See
{5.9} for more information concerning residual current
devices,
2. - the shell of the heater must
be bonded to the sheath or armour of the supply cable with
a conductor of current rating at least equal to the RCD
tripping current, but with a minimum cross-sectional area
of 2.5 mm².
C - Single-phase electrode heaters {Fig
7.15}
Fig 7.15 - Single-phase
electrode heater
The requirements for single-phase electrode
heaters are:
1. - a double-pole linked circuit
breaker with overload protection in each line,
2. - the shell of the heater bonded
to the sheath and/or armour of the supply cable with a conductor
of current-carrying capacity at least equal to that of each
live conductor,
3. - the shell of the heater bonded
to the neutral,
4. - the supply must be one which
has an earthed neutral.
D - Insulated single-phase electrode
heater, not permanently piped to water supply
Fig 7.16 Insulated single-phase
electrode heater, not piped in to a water supply
This small heater is usually fixed in position,
but is filled by using a flexible hose or a water
container.
The requirements are:
1. - there must be no contact with
earthed metal,
2. - the heater must be insulated
and shielded to prevent the electrodes from being touched,
3. - control must be by means of
a single-pole fuse or by a circuit breaker and a
----- double pole switch,
4. - the shell of the heater must
be bonded to the cable sheath.
