There are several reasons not to go into a data center's circuit-breaker panel, and to use a circuit tracer instead.
PDU Cables publishes Ken's Korner by Ken Koty, a former data center facility manager. While his advice is usually excellent, a column on data center power design and maintenance made a recommendation that could be dangerous.
How can you ensure that you are turning off the correct circuit for server downtime?
Ken wrote about the procedure for disconnecting equipment and removing power whips, and his recommendation was to test the circuit inside the breaker panel to see if current was flowing before shutting off the breaker. This is to ensure the circuit being shut off is really the unused one.
Ken discussed using an amp probe, or clamp-on meter, to trace the circuit for a piece of data center equipment from the breaker panel. But only a licensed electrician should go inside a live circuit-breaker panel, even if it's just to attach a clamp-on meter.
Attaching a clamp-on meter disturbs wires. If a wire in the panel happens to be loose, it could be dislodged, unintentionally and without warning downing a circuit. A licensed electrician should retighten all wires before any wires are moved to attach a meter.
Many breaker panels are mounted in power distribution units (PDUs) or power management modules, cabinets with multiple panels that can have current levels high enough to require Arc Flash Hazard warning placards. These warning placards aren't always installed where they should be. This is one reason why no one other than a licensed electrician should handle electrical gear. Also remember, in a data center, to work only with an electrician who is familiar with the installation and debriefed on the critical nature of a computing operation.
If arc flash is a possibility, properly shut down either the entire panel or PDU. The electrician may wear an arc flash hazard suit, which looks eerily similar to a space suit.
Circuit tracers are an easy-to-use alternative way to verify a circuit before shutting off the associated breaker. Simply plug the signal generator into the rack circuit to be deactivated, and scan the breakers in the panel with the receiver. The correct breaker is quickly, easily and safely identified and verified before shutting down the circuit.
Modern data center power best practices
Data center power design is evolving to help protect workers and equipment. In legacy data centers, where abandoned circuits must be removed, the approach described above is much safer for the operation than opening panels and clamping on meters.
With the power designs used in most modern data centers, there should be little need to remove old circuits. Dual circuits run to every rack and cabinet -- often in three-phase configuration -- installed in accordance with building codes. These dual circuits don't need to be removed if they are not used.
Power busway systems also allow circuits to be safely installed and removed without an electrician's help, via plug-in modules that make circuit identification obvious.
About the author:
Robert McFarlane is a principal in charge of data center design at Shen Milsom and Wilke LLC,
with more than 35 years of experience. An expert in data center power and cooling, he helped
pioneer building cable design and is a corresponding member of ASHRAE TC9.9. McFarlane also
teaches at Marist College's Institute for Data Center Professionals.
This was first published in September 2013