Type B Vital Relays - Alstom

Theory of Operation
2.5.5. Code-Responsive Relay
Type B code-responsive relays are built in Size 1 only. They are made with a light
armature and contact structure so that they respond quickly to pulses of coded energy,
even when the pulse frequency is as high as four pulses per second.
Two types of structures are available. One structure has a maximum of two dependent
front-back contacts and the other has a maximum of four dependent front-back
contacts. The four-contact structure is usually supplied for heavy service, as the contact
design provides more effective non-bounce characteristics.
The relay armature is polarized by two permanent magnets so arranged as to allow the
front contacts to close only if the proper polarity is applied. When the relay is deenergized,
a bias spring, together with magnetic bias, returns the armature to the deenergized
position to close the back contacts. Figures 2–31 and 2–32 show how the
relay works.
TRACK CIRCUIT DEENERGIZED
+ –
CODE TRANSMITTER
SHUNTING RAILS
N
S
Figure 2–31. Code-Responsive Relay De-energized
P1457, Rev. Nov/07 2–34 Alstom Signaling Inc.