28

Mechanical pressure switches

Pressure switch with locking of switching state (reclosing lockout)

Pressure switch with switching state

locking (reclosing lockout)

In the case of limiter functions, the switching state must be retained and locked, and it may be unlocked

and the system restarted only after the cause of the safety shutdown has been eliminated. There are two

ways of doing this:

1. Mechanical locking inside the pressure switch

Instead of a microswitch with automatic reset, limiters contain a "bi-stable" microswitch. If the pressure

reaches the value set on the scale, the microswitch trips over and remains in this position. The lock can

be released by pressing the unlocking button (identified by a red dot on the scale side of the switching

device). The lock can operate with rising or falling pressure, depending on the version.

The device can

only be unlocked when the pressure has been reduced (or increased) by the amount of the

predefined switching differential. When selecting a pressure limiter, it is necessary to distinguish

between maximum and minimum pressure monitoring. Ex-d versions cannot be equipped with internal

locking.

Maximum pressure limitation

Minimum pressure limitation

Switching and interlock-

Switching and interlocking

ing with rising pressure.

with falling pressure.

Additional function

Additional function

ZF205.

ZF206.

Connection of control

Connection of control

current circuit to

current circuit to

terminals 1 and 3.

terminals 2 and 3.

2. External electrical interlock in the control cabinet (suggested circuits)

A pressure monitor (microswitch with automatic reset) can also be used as a limiter if an electrical

interlock is added. For pressure limitation in steam and hot water boilers, an external interlock is only

permitted if it has been ascertained that the pressure monitor is "of special construction".

Maximum pressure limitation

Minimum pressure limitation

with external interlock

with external interlock

Where the above lock circuit is used, the requirements of DIN 57 116/VDE 0116 are met if the electrical

equipment (such as contactors or relays) of the external interlock circuit satisfy VDE 0660 or VDE 0435.