Banner EZ-SCREEN LPM Series - Machine Primary Control Elements and EDM Input

Single-Channel Control
Single-channel control uses a series connection of FSD contacts to form a safe switching point. After this point in the
machine’s safety-related control system, failures can occur that would result in a loss of the safety function (such as a
short-circuit to a secondary source of energy or voltage). For this reason, single-channel control interfacing should be used
only in installations where FSD safety stop circuits and the MPCEs are mounted within the same control panel, adjacent to
each other, and are directly connected to each other; or where the possibility of such a failure can be excluded. If this
cannot be achieved, then dual-channel control should be used.
Methods to exclude the possibility of these failures include, but are not limited to:
•
Physically separating interconnecting control wires from each other and from secondary sources of power
• Routing interconnecting control wires in separate conduit, runs, or channels
• Locating all elements (modules, switches, and devices under control) within one control panel, adjacent to each
other, and directly connected with short wires
• Properly installing multi-conductor cabling and multiple wires through strain relief fittings. Over-tightening of a
strain-relief can cause short-circuits at that point.
•
Using positive-opening or direct-drive components, installed and mounted in a positive mode
6.4.3 Machine Primary Control Elements and EDM Input
Each of the two Machine Primary Control Elements (MPCE1 and MPCE2) must be capable of immediately stopping the
dangerous machine motion, irrespective of the state of the other. A machine primary control element (MPCE) is an
electrically powered element that directly controls the normal operation of a machine in such a way that it is the last element
(in time) to function when machine operation is to be initiated or arrested (per IEC 61496-1). Examples include motor
contactors, clutch/brakes, valves, and solenoids.
Depending on the level of risk of harm, it may be required to provide redundant MPCEs or other control devices that are
capable of immediately stopping the dangerous machine motion, irrespective of the state of the other. These two machine
control channels need not be identical (diverse redundant), but the stop time performance of the machine (Ts, used to
calculate the safety distance, see Calculating the Safety Distance (Minimum Distance) (p. 19)) must take into account the
slower of the two channels. Refer to Generic Wiring to FSDs (manual reset) (p. 47) or consult the machine manufacturer
for additional information.
To ensure that an accumulation of failures does not compromise the redundant control scheme (cause a failure to danger),
a method to verify the normal functioning of MPCEs or other control devices is required. EZ-SCREEN LPM provides a
convenient method for this verification: external device monitoring (EDM).
For the EZ-SCREEN LPM external device monitoring to function properly, each device must include a normally closed,
forced-guided (mechanically linked) contact that can accurately reflect
the status of the device. This ensures that the
normally open contacts, used for controlling hazardous motion, have a positive relationship with the normally closed
monitoring contacts and can detect a failure to danger (for example, contacts that are welded closed or stuck On).
It is strongly recommended that a normally closed, forced-guided monitoring contact of each FSD and MPCE be connected
in series with the EDM input (see
Generic Wiring to FSDs (manual reset) (p. 47)). If this is done, proper operation will be
verified. Monitoring FSD and MPCE contacts is one method of maintaining control reliability (OSHA/ANSI) and Category 3
and 4 (ISO 13849-1).
If monitoring contacts are not available or do not meet the design requirement of being forced-guided (mechanically linked),
it is recommended you:
• Replace the devices so that they are capable of being monitored; or
• Incorporate the EDM function into the circuit as close to the MPCE as possible (for example, monitor the FSDs); and
• Employ the use of well-tried, tested, and robust components, and generally accepted safety principles, including
fault exclusion, into the design and installation to either eliminate, or reduce to an acceptable (minimal) level of risk,
the possibility of undetected faults or failures that can result in the loss of the safety function.
The principle of fault exclusion allows the designer to design out the possibility of various failures and justify it through the
risk assessment process to meet the required level of safety performance, such as the requirements of Category 2, 3, or 4.
See ISO 13849-1/-2 for further information.
WARNING:
• External Device Monitoring (EDM)
•
Creating a hazardous situation could result in serious injury or death.
• If the system is configured for “no monitoring,” it is the user’s responsibility to ensure this does
not create a hazardous situation.
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