ARMY TM 9-6115604-12
NAVY NAVFAC P-8-633-12
Details of Blocking and Tagging.
The supervisor is responsible for designating a qualified person to perform the switching, blocking, and
tagging. The technician who is to receive a Safe Clearance may be authorized to perform these operations.
The supervisor is also responsible for making any necessary arrangements for interruptions of service, such as
notifying electric power consumers (as required) and notifying the public utility supplying power to the
installation prior to the performance of switching which may affect the public company's system. This
information must be given to the person designated by the public utility to receive such information. In the
event this individual cannot be reached, the nearest system operating or load dispatching office of the utility is
to be Informed.
The technician designated by the supervisor to receive the Safe Clearance is responsible for filling in all details
of blocking, switching, and tagging operations In the space provided on the Safe Clearance form and for
having these details checked and approved by the supervisor. This is done in advance of the beginning of
switching. These details must be entered in their proper sequence, reading down the form. Included will be any
switch moves necessary to transfer load or put other equipment Into operation before deenergizing the lines or
equipment covered by the Safe Clearance
Blocking is defined as the placing of a prime mover valve, switch, or other line opening In the open or closed
position, as the case may be, and ensuring by mechanical means, or otherwise, that the position of the device
will not be changed accidentally.
Red tags are applied to devices to ensure that their position will not be changed by unauthorized persons as
long as lines or equipment are blocked out and red tagged under an active Safe Clearance. A red tag must be
applied for each Safe Clearance number and the name of the person to whom the Safe Clearance is to be
Issued must be shown. Refer to the examples in sub steps 1 through 4, below Yellow tags are used in
connection with Safe Clearances to show an abnormal position of a switch or other device (which does not
require a red tag) and ensure that its position will not be changed. Refer to the example in sub step 5, below .
Gang-operated switches must be locked open and a single red tag applied to the lock.
On overhead lines, a visible line break must be provided at all points of possible feed. An open oil circuit
breaker is not acceptable in lieu of visible line break on an overhead system and must not be used
except when it is not reasonable to provide a visible line break by removing the line side leads from the
oil circuit breaker bushing or otherwise. If used, the oil circuit breaker will be mechanically blocked or
locked open and red tagged and more than ordinary caution exercised in determining that the line is
deenergized. Grounds will be installed on the line as close as possible to the oil circuit breaker.
On underground systems, when it is not reasonable to provide a visible line break, an oil circuit breaker
or subway oil disconnect switch locked or blocked mechanically in the open position and red tagged is
acceptable, but the cautions cited in substep 2, above, for oil circuit breakers on overhead systems must
Oil fuse cutouts must be blocked or locked in the open position, the fuse block removed, and the clamp
A normally open switch which has been closed to tie two lines together, preparatory to taking a section of
one of the lines out of service on a Safe Clearance, is yellow tagged. Switches on the distribution system
which are normally open should be yellow tagged. However, if the position of a yellow tagged switch is
changed, the yellow tag must be removed and the proper spaces filled In by the person removing it. The
completed tag must be turned in to the supervisor.