it does not say the marking shall meet the
requirements in 110.21(B). That is because
110.21(B) is for caution, warning or danger
signs or labels. The marking required by
110.24(A) is not a caution, warning or danger marking (see Figure 4).
The informational note under
110.24(A) provides guidance as to why
the marking of the maximum available
fault current is necessary. The first sentence says the available fault-current
marking(s) addressed in 110.24 are
related to required short-circuit current ratings of equipment. Two of the
requirements pertaining to short-circuit
current ratings of equipment are in 110.9
In accordance with 110.9, equipment intended to interrupt current at
fault levels shall have an interrupting
rating at nominal circuit voltage sufficient for the current that is available
at the line terminals of the equipment.
In accordance with 110.10, circuit protective devices must be able to clear a
fault without extensive damage to the
electrical equipment. Therefore, the
overcurrent protective devices, the total
impedance, the equipment short-circuit
current ratings, and other characteristics
of the circuit to be protected must be
considered in order to select the correct
When service equipment is marked
with the maximum available fault current, it is easier to ensure the equipment’s
interrupting rating and the short-circuit
current rating is equal to or higher than
the available fault current. The second
sentence of this informational note says
NFPA 70E, Standard for Electrical Safety
in the Workplace, provides assistance in
determining the severity of potential
exposure, planning safe work practices,
and selecting personal protective equipment (PPE).
FIGURE 2 CT WITH NO AMMETER
It is possible to have high voltage produced on the secondary side of a CT if the
secondary terminals or conductors are not short-circuited. This potential can be
dangerous to personnel and can damage the CT.
Having an open secondary circuit while the transformer is energized is a Code
violation because it creates an unsafe condition.
CT with no ammeter in the secondary circuit
FIGURE 3 BEFORE REMOVING THE AMMETER
Before removing the ammeter from the circuit, install a jumper to short-circuit the
C T’s secondary-circuit conductors.
When an ammeter needs to be disconnected and removed from a C T, ensure you
short-circuit the CT’s secondary-circuit conductors first.
the C T
It is now safe
and remove the