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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

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)

on page 28) must take into account

the slower of the two channels. 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 LS provides a

convenient method for this verification: external device monitoring (EDM).
For the EZ-SCREEN LS 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 Diagram—8-pin Receiver and IM-T-9A Interface Module

on page 58 ). 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: EDM Monitoring. If the System is configured for “No Monitoring,” it is the user’s responsibility

to ensure that this does not create a hazardous situation. Failure to follow these instructions could result

in serious injury or death.

6.4.4 External Device Monitoring

EZ-SCREEN LS provides two possible EDM configurations: 1-channel monitoring and no monitoring. Their functions are

described below. The most common form of EDM is 1-channel monitoring; its primary advantages is the simplicity of wiring.

The installation must prevent short circuits across the N.C. monitoring contacts and to secondary sources of power.