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Type D-3 Direct Current Relay
Figure 7: Internal Schematic of the Type D-3 in the Type
FT-21 Case with Two Self-Contained Auxiliary
Relays used to Increase Contact Capacity.
connections may be made directly to the terminals by
means of screws for steel panel mounting or to the
terminal studs furnished with the relay for thick panel
mounting. The terminal studs may be easily removed
or inserted by locking two nuts on the stud and then
turning the proper nut with a wrench.
For detailed FT. case information refer to Instruction
Leaflet 41-076.
Standard D-3 relays are calibrated in millivolts mea-
sured at the relay terminals. Hence, the resistance of
the leads between the relay and the ammeter shunt
must be sufficiently low to avoid introducing an
excessive error in the relay indication. With the 0-10
or 5-0-5 millivolt relays, leads 8 feet long of 910 B&S
gauge copper wire will reduce the relay indication by
approximately 5%. if the lead length is less or the
conductor size is larger, the error will be correspond-
ingly reduced. Relays with higher millivolt ranges
have proportionally greater internal resistances, and
consequently error due to lead resistance is reduced.
For example, a 0-100 millivolt relay with leads as
described above would have an error of 0.5% due to
lead resistance.
The use of standard ammeter shunts may be
avoided by connecting the moving element leads
over an equivalent length of bus-bar or cable. On a
182A786
Figure 8: Internal Schematic of the Type D-3 Relay in the
Type FT-21 Case, Modified for ac Operation.
basis of 1000 amp. per square inch, at .20°C, 6 feet
of copper bus-bar will give 50 millivolts drop.
7.0 ADJUSTMENTS AND
MAINTENANCE
The proper adjustments to insure correct operations
have been made at the factory. Upon receipt of the
relay, no customer adjustments, other than those
covered under "SETTINGS" should be required.
7.1
ACCEPTANCE CHECK
Check the scale markings by setting either of the two
adjustable contacts at a value marked on the scale.
Then alternately apply this voltage plus and minus
3%. The contacts should make and break.
Remove the adjustable contact from the setting and
set the second adjustable contact at the same point
on the scale. Alternately -apply this voltage plus and
minus 3%. The contacts should make and break.
7.2
ROUTINE MAINTENANCE
All contacts should be cleaned periodically. A contact
burnisher S#182A836H0I is recommended for this
purpose. The use of abrasive material for cleaning
41-151.1H
182A788
5
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