Circuits that do not have hazardous potentials or hazardous energy levels and that do not directly control potential safety hazards are exempt from this section.
8.1.1 Fail-Safe Design. Circuits shall be so designed that a failure of any component in the system will prevent unsafe operation of the system. See 7.6.
8.1.2 Connection of Control Devices. Control devices shall be connected as follows:
1. Contacts shall not be parallel connected to increase ampacity.
2. Control device contacts shall open all ungrounded conductors of a circuit.
3. Indicator lamps and operating coils of control devices shall have one terminal directly connected to the grounded side of the circuit.
4. Switches, contacts and other control devices shall be connected to the ungrounded side of the circuit.
EXCEPTION: Motor overload relay contacts may be connected to the grounded side of the circuit if no part of the circuit extends beyond the control enclosure.
8.1.3 Emergency Off (EMO). Operator controlled equipment shall have an EMO control circuit which:
1. Shall remove all hazardous potentials outside the main electrical control enclosure within two seconds.
2. Shall stop all hazardous mechanical motion.
3. Shall not create any unsafe conditions.
4. Shall be designed to stop through de-energization rather than energization of circuits.
5. Shall take precedence over all start / stop controls. TEST: See item 4 under 7.6.
6. Shall comply with all the requirements for the EMO switch in 7.4.4.
8.1.4 Devices Allowed in the EMO Circuit. Interlocks, fire alarm switches, fume detectors, operator controls, temperature and other similar controls may be included in this circuit.
8.1.5 EMO Exceptions. The following equipment does not require an EMO circuit:
1. Equipment or units which operate independently of operator controls, e.g., water serving more than one station, ceiling suspended air showers, etc., do not require an EMO control.
2. Equipment which would require EMO control when in the stand alone mode, but is installed as a part of a system and is powered from a main electrical control enclosure with an EMO circuit, may only require an extension of the existing EMO circuit.
3. Self contained (free standing) units under 0.25 kVA.
8.1.6 Emergency Stop (EMS). Automated equipment and systems having hazardous mechanical interfaces may require unique and special EMS controls which remove the physical hazard at the interface but do not shut off the associated equipment. EMS controls:
1. Shall be accessible to the person located at the hazardous interface position.
2. Shall stop all hazardous mechanical motion at the equipment interface.
3. Shall not create any unsafe conditions.
4. Shall be designed to stop through de-energization rather than energization of circuits.
5. Shall take precedence over associated start controls. TEST: See item 4 under 7.6.
6. Shall comply with all the requirements for the EMO switch in 7.4.4.
EXCEPTION: The label shall read: "EMERGENCY STOP"
8.1.7 Multiple Sources of Power.
Equipment with multiple sources of power, permitted by 3.10.1, shall comply with all of the following requirements:
1. Each source of power shall have a separate main electrical control enclosure. See 6.2.6.
2. Each main electrical control enclosure shall:
a. Be clearly and permanently labeled.
b. Have a supply circuit disconnecting means that is interlocked with the enclosure door and de-energizes all hazardous potentials and hazardous energy levels within that enclosure.
c. Have an EMO that de-energizes all hazardous potentials and hazardous energy levels leaving that enclosure.
d. Have interlock circuit such that loss of power from any source will trip the EMO circuit.
3. Hazardous potentials from each main electrical control enclosure shall be contained in separate conduit, cables, enclosures and junction boxes.
4. Control circuits shall:
a. Be supplied by the 60 Hz power source.
b. Have nonhazardous potentials.
5. The equipment shall have an EMO circuit that complies with 8.3.
Single-phase equipment operating at 120 V or less with up to 2 kVA main protection.
8.2.1 Controls. Controls in small equipment may be at line voltage but shall comply with the requirements of Sec. 7.
8.2.2 Emergency Off (EMO).
Small equipment shall have an EMO control that complies with one of the following:
1. The main supply disconnect switch may be used if it complies with the following requirements:
a. Readily accessible to
the equipment operator.b. Clearly labeled "EMERGENCY OFF."
c. Clearly indicates ON / OFF status.
2. A dedicated EMO switch may be used if it complies with 7.4.4.
8.3 Large Equipment and Systems
- Multiphase equipment and equipment operating at 120 V with 2 kVA or more main protection.
- Two or more units connected together to form a system.
8.3.1 Required Control Circuits. Control circuits in large equipment and systems shall comply with the following:
1. Equipment and systems shall have an EMO control circuit.
2. Equipment and systems shall have a power circuit.
Notes:
1. See Figure 28.
2. EMO circuits and power control circuits at the same voltage may be combined.
3. An EMO contactor shall be utilized to disconnect the hazardous potentials and hazardous energy levels inside the electrical enclosure when the EMO switch is operated.
8.3.2 Circuit Control Limits. Circuit control limits for large equipment and systems are:
1. EMO control circuit shall not be over
24 V, nominal, or 240 VA.
2. Power control circuits shall not be over 120 VAC or 60 VDC.
Notes:
1. To minimize exposure to operators and service personnel, all control circuits should be limited to 24 V.
2. Circuits over 24 V shall be designed, built, installed, enclosed, grounded, protected, and inspected for strict compliance with this standard.
8.3.3 AC Control Circuits. AC control circuits in large equipment and systems shall be derived from a control transformer that complies with all the requirements of 3.6 and the following:
1. Shall be sized to support the VA in rush of the circuit components. See Table 6 and Table 9.
2. Shall be connected to the load side of the main supply disconnect.
8.3.4 DC Control Circuits. DC control circuits in large equipment and systems shall comply with the following:
1. Shall be designed for complete electrical isolation between AC and DC circuits.
2. Shall be properly protected for overcurrent.
3. Shall be powered from the load side of the main supply.
8.3.5 Emergency Off (EMO). EMO controls in large equipment and systems shall comply with 7.4.4.
Note: Multiple switches are required when the equipment / system size or geometry is such that a single switch does not provide adequate accessibility for operators or equipment service personnel.
8.4 System Circuit Requirements
8.4.1 Units Mounted in or on Other Units. Units in this type of system:
1. Shall have the same source of power,
see 3.10.1.
2. Shall have the same supply circuit disconnecting means. See 3.10.7.
3. Shall have the same EMO circuit.
8.4.2 Multiple Units with Interconnecting Circuits. Units in this type of system with hazardous potentials:
1. Shall have the same source of power,
see 3.10.1.
2. Shall have the same supply circuit disconnecting means, see 3.10.7.
3. Shall have the same EMO circuit,
8.4.3 Multiple Units Mounted Separately. Multiple units mounted separately with NO interconnecting circuits with hazardous potentials or energy levels:
1. May have separate sources of power.
2. May have separate supply circuit disconnecting means.
3. May have separate EMO circuits,
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