Overview
For maximum safety, our manuals recommend de-energizing live circuits before installing current transformers (CTs). However, this is not always necessary.
In all cases, only a licensed electrician should install current transformers and meters. All NFPA and NEC rules should be followed.
The following discussion lists and summarizes some of the relevant NFPA articles. However, there are other relevant articles not mentioned and our interpretation of these articles may be incorrect or incomplete. CONSULT AN NFPA EXPERT BEFORE INSTALLING CURRENT TRANSFORMERS WITHOUT DE-ENERGIZING THE CIRCUIT!
NFPA 70E
According to the National Fire Protection Association (NFPA), 70E: Electrical Safety in the Workplace (2009).
- Article 130.1 Justification for Work: exposed conductors and circuit parts must be put into a safe condition (de-energized) before working on the circuit.
- Article 100 Definitions: “Exposed (as applied to energized electrical conductors or circuit parts). Capable of being inadvertently touched or approached nearer than a safe distance by a person. It is applied to electrical conductors or circuit parts that are not suitably guarded, isolated, or insulated.”
Therefore, if the conductor to be monitored by the CT is insulated, it is not “exposed”. The CT is fully insulated except for the ends of the output wires, which are nominally 0.333 Vac (below the 50 volt limit of 130.1.(A).(3)) and only generate milliamps of current, so it cannot generate an arc.
Therefore, it appears the NFPA allows installing a CT on an insulated energized conductor, provided all the safety rules of NFPA 70 and 70E are followed.
Special Cases
Article 130.1 furthermore allows working on exposed energized circuits where de-energizing the circuit would pose “Greater Hazard” or is infeasible. See the standard for the exact rules under which this is allowed. This could allow for installing a current transformer on a live uninsulated conductor, such as a busbar. In this case, a written work permit is required. If this type of installation is necessary and can be performed according to NFPA rules, CCS reiterates the importance of proper training, arc-flash safety equipment, and insulated gloves.
UL 2808
The UL 2808 Outline of Investigation for Energy Monitoring Current Transformers Issue Number: 3 June 10, 2016 states in section: “9.3 Field wiring leads of indoor-use current transformers shall be suitable for contact with exposed live parts of a Class 1 circuit and/or routing through the same conduit with other Class 1 circuit conductors. The conductors shall consist of Class 1 wiring…”.