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7/11 jokab safety
ABB Low Voltage Industrial Products 0312
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Safety relays
Why should you use safety relays?
A fault in the control circuit logic, or failure of or damage to the
control circuit must not lead to dangerous situations. This is the
requirement in the EU´s Machinery Directive 98/37/EC under
the heading 1.2.7. “ Failure of the control circuit”. The directive
implies that no person should be put at risk if for example, a
relay sticks or if a transistor or two electrical conductors short-
circuit.
A safety relay will fulfill these requirements. A safety relay
has, for example, inputs that are checked for short-circuits and
for safe stops and reliable restarts
Dual stop signals when the gate
is opened
.
Entering or putting a hand or limb into a
hazardousareamustcauseallmachinery
that can cause personal injury to stop
safely. Many serious accidents occur
when machinery is believed to have
stopped but is in fact only pausing in
its program sequence. The safety relay
monitors the gate interlock switch and
cables and gives dual stop signals.
Supervised reset when there can be
a person within the risk area.
To make sure that nobody is within the
restricted area when activating the reset
button. A supervised reset button must
be pressed and released before a reset
can occur. Many serious accidents have
been caused by an unintentional and
unsupervised reset.
Timed reset when you cannot see
the entire risk area
.
Sometimes a double reset function is
necessary to make sure that no one is
left behind in the risk area. First, after
ensuring no other person is inside the
hazardousarea,thepre-resetbuttonmust
be activated, followed by the reset button
outside the risk area within an acceptable
time period e.g 10 seconds. A safety
timer and a safety relay can provide this
function.
Automatic reset for small hatches.
Where body entry is not possible
through a hatch, the safety
circuit can be automatically reset.
The safety relays are reset immediately
whenthehatch interlockswitchcontacts
are closed.
dual redundant circuits that are checked at each operation. This
can be compared to the dual brake circuits in a car. If one of the
circuits is faulty the other will stop the car. In a safety relay there
is an additional function which only allows a machine to start if
both circuits are ok.
The standard for safety related parts of the control system
describes various safety categories depending on the level of risk
and application. One single universal relay with selectable safety
categories solves this.
to supervise safety devices
Light beams Light grids/curtains 3-position
devices
Sensors/
switches
Two-hand
controls
Emergency
stops
Strips
Mats
to meet existing safety standards