TYPICAL
THERMAL SEQUENCING
(HEAT) RELAY
M1
M2
H
H
FIGURE 17
PRIMARY LIMIT S15,
SECONDARY LIMIT S20
Page 29
8-Thermal (Heat) Relay K32, K33
Thermal sequencing (heat) relays are used to energize
heating elements in all ECH16R electric heaters. A heat
relay is a normally open relay with a resistive element for
a coil and a bimetal disk actuating the contacts. When
the relay is energized, internal resistance heats the biĆ
metal disk causing the contacts to close. When the relay
is deĆenergized, the disk cools and the contacts open.
A sequencing relay has
multiple contacts. Each
set of contacts is conĆ
nected to a separate biĆ
metal disk. When the
relay is energized, interĆ
nal resistance heats the
bimetal disks at different
rates causing the conĆ
tacts to close at different
times.
The contacts are calibrated to operate on a first on last
off basis. Some heat relays may have two resistive coils;
each with its own set of contacts. Although they are
physically attached, they operate as two independent
relays. Figure 16 shows a typical heat relay.
9-Fuse F3
F3 is a current limiting fuse connected in series with
each leg of electric heat. See tables 2 or 3 for fuse ratĆ
ings.
10-High Temperature Primary S15
S15 is the primary high temperature limit. It is located in
the electric heat unit immediately downstream from the
heating elements. S15 is a singleĆpole singleĆthrow norĆ
mally closed thermostat. S15 ratings are printed on side
of limit.
In ECH16R series
units, multiple S15 therĆ
mostats may be used.
One S15 thermostat is
wired in series with
each heating element.
When S15 trips, some
heating elements may
remain operable.
In ECH16 commercial units, only one S15 thermostat is
used. A single S15 thermostat is wired in series with pilot
relay K43. When S15 trips, all heating elements are deĆ
energized.
11-High Temperature Secondary S20
ECH16 commercial units use nonĆresettable thermoĆ
stats for secondary high temperature protection. One
switch is connected in series with each leg of the eleĆ
ment assembly. Each S20 thermostat is physically loĆ
cated adjacent to the element it is protecting. S20 is a
singleĆpole singleĆthrow normally closed thermostat.
The thermostat actuates at 185°F +
8°F on a temperaĆ
ture rise and cannot be reset. Once tripped, it must be
replaced.
12-Heating Elements
ECH16 and ECH16R heating elements are composed
of helix wound nichrome wire heating elements which
are exposed directly to the airstream. Heating elements
are energized directly by contactors or thermal (heat) reĆ
lays located on the control panel. Once energized, heat
transfer to the airstream is instantaneous. OvertemperĆ
ature protection is provided by primary and secondary
high temperature limits. Overcurrent protection (ECH16
commercial units only) is provided by current limiting
fuses.
13-Time Delay DL2
Time delay DL2 is an electronic single-pole single-
throw time delay relay used in 20 and 25kW three
phase ECH16/20 units. DL2 allows staging by providĆ
ing a timed interval between the first and second sets
of heating elements. DL2 is energized with contactor
K15 when pilot relay K43-1 closes. When the delay
coil is energized, it's normally open contacs are
delayed 30 seconds before closing. When the delay
coil is de-energized, its contacts are delayed one secĆ
ond before opening.
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