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SEMI S22-1103a © SEMI 2003, 2005 19 12.3.10 Machine parts, other than accessories or at tachment s, having metal-to-metal bearing surfaces sh ould be considered as bonded to the protective earthing system. Parts separate…
SEMI S22-1103a © SEMI 2003, 2005 18
11.14 Some device loads vary widely depending upon their operating conditions and operating history (such as
ceramic resistive heating elements). The load values used to select overcurrent protection devices should account for
the full load range during worst case, normal operating conditions.
12 The Protective Earthing System
12.1 Bonding All accessible non-current carrying conductive components that may become energized under
reasonably foreseeable single fault conditions, and as a result increase the risk of electric shock, should be bonded to
the protective earthing system.
NOTE 34: Non-safety related bonding conductors (e.g., bonding conductors for EMC concerns) are not subject to this section.
12.2 Parts Not Bonded to the Protective Earthing System If non-current carrying conductive parts do not
constitute a hazard under single fault conditions, it is not necessary to bond them to the protective earthing system.
12.3 Protective Earthing System
12.3.1 General The protective earthing system consists of those components that provide electrical continuity
between the protective earthing conductor terminal and non-current carrying frames, enclosures, or components that
are stipulated to be maintained at an earth potential under both normal operating conditions and single fault
conditions in accordance with ¶12.1.
12.3.2 Protective Conductors Copper conductors or structural members with bonding jumpers should be used for
bonding to the protective earthing system.
EXCEPTION: Conductors other than copper that have equivalent ampacity to copper conductor can be used for
protective conductors.
12.3.2.1 When a protective conductor is used in this way, its size should be compatible with the protective
conductor ampacity given in Tables A1-1 through A1-5 that is a function of the ampacity of the current carrying
conductor it is associated with.
12.3.2.2 Bus bars, and/or the equipment frame may be used as protective conductors. When bus bars or the
equipment frame is used, their geometry and material should provide the same ampacity as that stipulated for the
protective conductor in Tables A1-1 through A1-5.
12.3.3 Hinges, slides, and other moving parts of enclosures should not be relied on as part of the protective earthing
system.
EXCEPTION: Hinges are acceptable if the enclosure and its hinges have been identified as suitable for bonding by
an accredited testing laboratory.
12.3.4 Raceways, wireways, and cable trays should not be used as protective conductors, except to effect their own
connection to the protective bonding circuit as may be stipulated by ¶12.1.
12.3.5 It is preferred that a protective conductor be routed with the associated current carrying conductors whenever
possible.
12.3.6 The protective earthing system should not be intentionally used as a current carrying conductor, except as
permitted by ¶9.2.6.
12.3.7 Continuity of the Protective Earthing System The protective earthing system should be permanent,
electrically continuous and capable of carrying any ground fault current likely to be imposed.
See ¶22.3 “Earthing
Continuity and Continuity of the Protective Bonding Circuit” for the appropriate test method.
12.3.8 Connection and bonding points to protective conductors should be designed so that their current carrying
capacity is not impaired by mechanical, chemical, or any other influence that may degrade their current carrying
capacity.
12.3.9 Mounting hardware and cover screws that may be removed for normal servicing should not be used for
terminating the protective conductor to a component or part of the enclosure.
SEMI S22-1103a © SEMI 2003, 2005 19
12.3.10 Machine parts, other than accessories or attachments, having metal-to-metal bearing surfaces should be
considered as bonded to the protective earthing system. Parts separated by a nonconductive fluid or gas should not
be considered as bonded.
12.3.11 When a part is removed, the continuity of the protective earthing system for the remaining parts should not
be interrupted.
12.3.12 Exclusion of Switching Devices from the Protective Bonding Circuit The protective earthing system
should not incorporate switching or interruption devices.
EXCEPTION: For technologies requiring isolated potentials (i.e. no ground reference) during operation, ground
interrupting devices are acceptable provided the device automatically provides continuity of the protective earthing
system when the related components are accessible and that the device is fault tolerant.
12.3.13 Interruption of the Protective Earthing System Where the continuity of the protective earthing system
can be interrupted by means of connectors (e.g., plug and socket connections), and this could result in increased risk
of electric shock, the following considerations should be satisfied:
a) the protective earthing system should be interrupted only after the energized conductors have been
interrupted, and
b) the continuity of the protective earthing system should be re-established before any energized conductor is
reconnected.
12.4 Bonding Terminations to The Protective Earthing System
12.4.1 The protective conductors should be bonded to a single designated termination (the protective earthing
conductor terminal) that will not be disturbed by any other conductor terminations.
12.4.2 All non-conductive coatings, such as paint or enamel, should be removed from contact surfaces where
protective conductors terminate.
12.4.3 When terminal lugs are used for bonding to the protective earthing system, they should be ring-tongue type,
with lock or star washers. (Refer to ¶17.2(a).)
12.4.4 Terminal lugs used to make connections to the protective earthing system do not need to be insulated.
12.5 Cord and plug connected equipment should pass a leakage current test in accordance with the test described in
§22.
13 Safety Circuits
13.1 General — Transformers or power supplies should be used for supplying safety circuits. Such transformers
should have electrical isolation between the primary and the secondary windings. Safety circuits should be designed
using non-hazardous voltage and power levels consistent with the correct operation of the control circuit.
NOTE 35: See ¶13.3.3 Exception 2 for specific EMO function exception.
13.2 Start Functions
13.2.1 Start functions should operate by energizing the relevant circuit.
13.2.2 Prevention of Unexpected Start-up The engineering design should prevent unexpected start-up.
13.3 Emergency Off
13.3.1 The equipment should have an “emergency off” (EMO) circuit. An EMO actuator (e.g., button), when
activated, should place the equipment into a safe shutdown condition, without generating any additional hazard to
personnel or the facility.
EXCEPTION 1: Cord- and plug-connected single phase equipment, rated no greater than 240 Volts line-to-line/150
Volts line-to-earth and no greater than 2.4 kVA, where the hazards are only electrical in nature, do not need to have
a separate EMO circuit if the main disconnecting means is readily accessible to the operator and maintenance
personnel. This main disconnecting means should be red and labeled indicating its On/Off status.
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EXCEPTION 2: Assemblies that are not intended to be used as stand-alone equipment, but rather within an overall
integrated system, and which receive their power from the end-user system, may not have a separate emergency off
circuit. The assembly’s installation manual should provide clear instructions to the equipment installer to connect
the assembly to the integrated system’s emergency off circuit.
NOTE 36: It is recommended that the emergency off function not reduce the effectiveness of safety devices or of devices with
safety-related functions (e.g., magnetic or braking devices) necessary to bring the equipment to a safe shutdown condition
effectively.
13.3.2 EMO Interfaces External EMO interfaces should be provided where the equipment is likely to be
integrated and is likely to have shared hazards with other assemblies in the end user’s facility. If an external EMO
interface is provided, the supplier should include instructions for connecting to the interface.
13.3.3 EMO Function Activation of the emergency off circuit should de-energize all hazardous voltage and all
power greater than 240 volt-amperes in the equipment beyond the main power enclosure.
EXCEPTION 1: A non-hazardous voltage EMO circuit (typically 24 Volts) may remain energized.
EXCEPTION 2: Safety related devices (e.g., smoke detectors, gas/water leak detectors, pressure measurement
devices, etc.) may remain energized from a non-hazardous power source.
EXCEPTION 3: A computer system or PLC performing data/alarm logging and error recovery functions may
remain energized, provided that the breaker and receptacle supplying the power to the computer system are clearly
labeled as remaining energized after EMO activation.
NOTICE: ¶13.3.4 below will be withdrawn upon July 1, 2006 publication and replaced by the new ¶13.3.4
including: figures and tables as shown in Delayed Revisions §2, however, the EH & S Committee has voted
that implementation of the information is OPTIONAL before the effective date.
13.3.4 EMO Design The design of the EMO circuit should include all the following:
a) the EMO circuit should not include controls that enable it to be defeated or bypassed;
b) the EMO circuit should consist of electro-mechanical components;
c) resetting the EMO switch should not re-energize circuits, equipment, or subassemblies that create a hazard to
personnel or the facility;
d) the EMO circuit should shut down the equipment by de-energizing rather than energizing control components;
and
e) the EMO actuator should be non-lockable and self-latching.
EXCEPTION 1: Solid-state devices and components may be used, provided the system or relevant parts of the
system are evaluated and found suitable for use. The components should be evaluated and found suitable
considering abnormal conditions such as over voltage, under voltage, power supply interruption, transient.
NOTE 37: For equipment intended for use in locations where fire or explosion hazards may exist, it is recommended that a
pneumatic or intrinsically safe EMO circuit be considered.
13.3.5 EMO Identification The EMO identification should include the following:
a) the emergency off actuator should be red and mushroom shaped;
b) a yellow background for the EMO should be provided;
c) all Emergency Off actuators should be clearly labeled as “EMO,” “Emergency Off,” or the equivalent, and
should be clearly legible from the viewing location. The label may appear on the actuator or on the yellow
background; and
d) Emergency Off buttons should be located or guarded to minimize accidental activation.
13.3.6 EMO Location and Size Emergency Off buttons should be readily accessible from operating and regularly
scheduled maintenance locations and appropriately sized to enable activation by the heel of the palm.
13.3.7 No operator or regularly scheduled maintenance location should require more than 3 meters (10 feet) travel
to the EMO button.