Electrical power quality and distribution are an important facet of live sound reinforcement. No matter the size of the PA system, there always seems to be need for an additional AC outlet to plug into.
Power quality is also of importance as spikes (over voltages) and sags (under voltages) can wreak havoc or even damage and destroy gear.
The first place to look for information about power is the National Electric Code (NEC), published every three years and sponsored by the National Fire Protection Association (NFPA). The purpose of the NEC is to safeguard people and property from electrical hazards caused by improper electrical wiring or unsafe practices.
The actual code issued by NFPA is not a law, but rather to serve as an advisory document that outlines best/safe practices and offers warnings of known dangers. Note that local municipalities can (and many do) legislate and adopt the current version of the NEC as law.
Most venues have power service built in, but as production providers we still need to check it to ensure that it’s safe, sufficient and in good working order. It doesn’t matter if a journeyman electrician or an experienced show master electrician has wired up the power distribution (PD) or pointed to a “working” outlet, before plugging in any gear, pull out a meter and test the circuit.
With outlets, have a look to see if pieces are broken off or it looks like it’s burned. Avoid using any outlet if it appears damaged. Next, meter the outlet between the hot (smaller slot) and neutral (bigger slot), and there should be between 110 to 120 volts. This number can vary depending on how far the outlet/breaker box is from the utility pole/utility transformer.
Then test with the meter between the hot and ground (the rounded U-shaped hole under the two slots on a standard outlet), and there should be the same voltage reading as above. Next, check between the neutral and ground – there should not be any voltage. If there is, it means the outlet is wired incorrectly and is therefore dangerous, and should not be used.
This is one of the main reasons to employ a meter rather than a 3-light “cube” outlet tester. They can sense if an outlet has power and a ground but not if the outlet is wired “Reverse Polarity Bootleg Ground” (RPBG), which is dangerous.
Another quick way to check the safety of an outlet is to use a non-contact voltage tester or “volt tick,” which measures if there is voltage present. Note: test a volt tick on a known live circuit first to make sure it’s working. Also be sure to meter all outlets, including those on PD systems, as well as any generator outlets (if applicable). Related, check that the generator has a properly installed ground rod.
Once all outlets have been tested, equipment can be plugged in. Of course, the outlets are never located where the gear needs to be so extension cords are often required.
The NEC has something to say about portable power cords regarding their use in places where the public gathers. NEC Article 520.68 states: “Flexible conductors, including cable extensions, used to supply portable stage equipment shall be listed extra-hard usage cords or cables.”
It also goes on to state that hard-usage cords can be used on stand lights in locations where the cable is not subject to physical damage.
For outdoor events like concerts or fairs, NEC article 525.20 states: “Where flexible cords or cables are used, they shall be listed for extra-hard usage. Where flexible cords or cables are used and not subject to physical damage, they shall be permitted to be listed for hard usage. Where used outdoors, flexible cords and cables shall also be listed for wet locations.”
Manufacturers code cables with different numbers and ratings depending on their usage. In show business we normally use cables that have stranded inner conductors (so they are flexible enough to coil up after the gig) surrounded by insulation and outer rubber jackets.
These are usually designated as “S” service grade rated at 600 volts and “SJ” or Junior Service grade rated at 300 volts. The extra thick insulation and jacket thickness equates “S” cables as “extra-hard usage” and “SJ” cables fall under the “hard use” categories.