Hardware Manual

Saia-Burgess Controls AG

Hardware Manual PCD 1 / PCD 2 Series│Document 26 / 737│Edition EN 15│2008-07-22

Appendix A

A

A-8

Installation direction and relays contact protection

A.4.3 Switching inductive loadsSwitching inductive loads

Because of the physical properties of inductive loads, it is not possible to disconnect

inductance without interference. This interference must be minimized as far as pos-

sible. Although the PCD is immune to this interference, there are other devices which

may be susceptible.

It should be noted here that, as part of the harmonization of standards throughout the

EU, EMC standards have applied since 1996 (EMC Directive 89 / 336 / EG). Two firm

principles can therefore be stated:

● PROTECTION AGAINST INTERFERENCE FROM INDUCTIVE LOADS IS MAN-

DATORY

● INTERFERENCE SHOULD BE ELIMINATED AS CLOSE AS POSSIBLE TO ITS

SOURCE

Relay contacts on the present module have been wired. However, it is still recom-

mended that a protection circuit should be fitted at the load.

(Often available as normal components on standardized contactors and valves).

When switching direct voltage it is urgently recommended that a recovery diode is fit-

ted above the load. This should even take place when, theoretically, an Ohmic load is

switched. In practice, there will always be a proportion which is inductive (connection

cable, resistance coil, etc.). In this case it should be noted that the switch-off time will

be longer.

(Approximate Ta. L / RL * √ (RL * IL / 0,7).

For direct voltage, transistor output modules are recommended.

A.4.4 Relay manufacturer’s information on RC unit dimensioning

Wiring contact protection:

The purpose of contact protection wiring is to suppress switch arcing (“sparks”) and

thereby prolong the lifetime of the contacts. All protection wiring has disadvantages

as well as advantages. For the cancellation of arcing by means of an RC unit, see

adjacent diagram.

When switching off load circuits with inductive components (e.g. relay coils and

magnet coils) the interruption of current results in overvoltage (standard inductance)

at the switching contacts. This may amount to many times the operating voltage and

so threaten the insulation of the load circuit. The resultant breaking spark leads to

rapid wear of the relay contacts. For this reason contact protection wiring is particu-

larly important with inductive load circuits. Values for the RC combination can also

be determined from the adjacent diagram. However, for voltage U it is necessary to

use the overvoltage arising from the interruption of current (e.g. measurable with an

oscillograph). Current must be calculated from this voltage and the known resistance,

against which the voltage was measured.

Screening units should only use anti-interference capacitors that comply with VDE

0565 T1 class X2. These capacitors are switchproof and designed for particularly

high switching surges. They can also run directly on mains voltage.