MIL-HDBK-1025/10
Section 6. SUBSTATIONS AND SWITCHGEAR
System Familiarity. Comply with the requirements of OSHA standard
6.1
29 CFR 1926.269(u), "Substations." A substation provides a protected area where
equipment and lines permit switching power circuits and may allow transforming power
from one voltage to another. A substation presents a potential safety hazard because
usually only portions of the apparatus concerned can normally be de-energized. For safe
operation, a thorough knowledge of the system, including aerial and underground line
connections, is necessary. Systems are designed to be safe to operate if maintained
properly. Operating safely requires maintenance to be done in a manner that eliminates
risks and requires knowledge of the work area, its hazards, and its design operating
rationale.
6.1.1 Diagrams and Schematics. Electrical diagrams and schematics of the
substations should be available at the facility's engineering office and should be
continuously updated. Diagrams and schematics should be studied to understand the
operation of the systems and the location and connections of all circuits. Protective
devices, alarms, and interlocking circuits all operate to protect the system. The worker
should understand where, why, how, and when blocking protective devices will maintain
safe working conditions. However, only a supervisor can authorize blocking.
6.1.2 Engineering Guidance. Diagrams and schematics should be kept up to
date under the supervision of the facility's engineering staff. Staff guidance should be
sought when performing maintenance on complex systems. Staff input is mandatory if the
maintenance work involves additions or changes to the power and control systems
involved.
6.1.3 System Operation. System single line diagrams should be permanently
mounted at each substation. When lockout/tagout/tryout switching operations are
performed, mimic buses on switchgear are helpful as a visual indication of the lines or
equipment served.
6.1.3.1 Protective Devices. Protective devices within the system, such as relays
and fuses, which are to be worked on or replaced, should retain respectively their correct
coordination settings or be of the proper size and type. Always record previous data so
that changes in system coordination are not made.
6.1.3.2 Alarms. System alarms, if blocked during maintenance, should be
returned to their correct operating conditions.
6.1.3.3 Interlocking. Interlocking is provided to maintain proper electrical
operation in the case of a circuit loss or switching change. Interlocking provisions should
be known so as to eliminate any dangers of electrical feedback from another source,
possible paralleling of two unsynchronized sources, or other unsafe operations.
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