Pinnacle Systems NSD - Manual (Page 6)

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Overview
NSD Safety Mat System
Overview
The NSD Safety Mat System contains two independent
control circuits which allow for shut down due to any
single failure in one of the control circuits. The N.O.
output circuit is controlled by two captivated contact
relays in series which are both monitored. If the contact
in one relay should stick closed (i.e., contact weld) the
other relay will shut down and an LED will indicate the
fault, even if the contact on the bad relay opens back up
you must reset the power to clear the fault. The relay
contact monitoring provides a high degree of user safety.
Circuit Description
The NSD-TR-01 (metal box) and NSD-DR-01 (basic
DIN-rail) are alike in that they use discrete logic circuits
to detect the mat. The NSD-DR-04 (DIN-rail with
diagnostics) is diverse redundant microprocessor driven
and has a built-in diagnostics display. A low voltage
(24VAC) signal is sent out to the safety mat(s) through
two wires. The voltage passes through the mat and back
into the NSD control unit through two more wires. Two
special optically-isolated diverse redundant circuits look
at the signals coming back from the mat(s) detecting the
presence of proper positive and negative voltage (AC)
levels and output their signals to redundant missing
pulse detectors. If the voltage coming back is too high
or low, the missing pulse detectors will drop out and
de-energize the relay circuit. If the voltage coming back
is not AC (missing either positive or negative peaks),
then the signal going to the missing pulse detector will
disappear which will, in turn, drop out and de-energize
the relay circuit.
Redundant relay position detectors watch for a relay
contact stuck in the wrong position. The circuits will lock
out the reset feature of the NSD control unit if a relay
contact is closed when it should be open.
Each relay has its own driver circuit to complete the
redundancy of the circuits.
The output relays will de-energize if the voltage coming
from the mat(s) is either too high or low (12VAC to
36VAC) or is not AC (i.e., DC), including the following
reasons:
1. When you step on the mat, you are shorting
out the 24VAC signal going back into the NSD
control unit to OV, so the relays de-energize.
2. If you cut any wire going to or coming from
the mat(s), the signal going back into the NSD
control unit is OV, so the relays de-energize.
3. If any wire going to or coming from the mat(s)
is cut and/or shorted to any other mat wire, the
signal going back into the NSD control unit is
OV, so the relays de-energize.
4. If any wire going to or coming from the mat(s)
is cut and/or shorted to an outside AC power
source (i.e., 120VAC) the signal going back
into the NSD control unit will exceed the 24VAC
signal level and the relays will de-energize.
5. If any wire going to or coming from the mat(s)
is cut and/or shorted to an outside DC power
source (i.e., 24VDC), the signal going back into
the NSD control unit will not be an AC signal
and the relays will de-energize.
6. If, when you step on the mat, either safety output
relay is stuck in the closed position, the NSD
control unit will automatically shut down the
other relay and lock out and further attempts
to reset the unit until the power is reset.
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