Cpa Relay Special Considerations - Tadiran Telecom FlexSet 280 Manual

5
DC Arc
Figure 10-7
Suppression for CPA Relay

CPA Relay Special Considerations

The relay contacts provided on the CPA module have specific electrical limitations which are
described in detail in Specifications on page 10-13. These limitations must not be exceeded. In
addition, the type of load the relay contact operates can have a great significance.
Ideally, the contact should operate a low voltage, low current, non-inductive load, such as solid
state device. When the voltage or current increases, or when an inductive load, such as a relay
coil or solenoid is connected, the likelihood of arcing across the contact increases. Contact
arcing occurs while the contact is opening, and burns away the precious metal surface,
increasing the chance of the contact being "welded" closed or failing completely. Arcing also
generates high frequency energy which may be coupled to any one of the data busses of the
Coral, causing unreliable or unpredictable system behavior.
If the contact is to operate a relay or solenoid, use an alternating current (AC) operated device,
and an AC power source, if at all possible. AC operated inductive loads produce much lower
arc levels than direct current (DC) operated loads.
If the contact must operate a DC powered inductive load, the contact must be adequately
protected by arc suppression devices. Figure 10-7 illustrates the use of a common rectifier
diode as a simple arc suppression device. The diode is normally reverse biased (not conducting)
with the cathode (top side in the drawing) connected to the positive side of the power source.
Coral
system
CPA
When the inductive load is powered, a magnetic field is created around the coil windings.
When power is removed, the magnetic field quickly deteriorates, producing a reverse polarity
voltage (counter EMF) across the windings. This reverse polarity voltage is typically many
times the voltage which originally powered the coil, providing enough potential to arc across
the open relay contacts.
By placing the diode in the circuit, the reverse polarity current is conducted by the diode,
shunting the energy around the contacts. A rectifier rated at 1.0 ampere forward current (1.0A
I ), and a peak inverse (reverse) voltage of at least 250 volts (250V PIV, PRV, or V
F
be sufficient in most cases.
2
Relay
5
(+)
DC
power
supply
Relay
(-)
Spike
suppression
diode
) should
RM
Coral Terminal Equipment
10-7