[BC] Transmitter Safety

[RichardBJohnson at comcast.net]

Broadcast transmitters are not required to be comply
with the NEC code although many/most/all certainly
do because anything behind an interlock can be
scatter-wired as long as HV and low voltage wiring
is isolated.

Many 3-phase services are 208-wye. This gives
you 120 volts to the neutral-connected common.

If you use such a circuit, the ampacity of all
the wires needs to be such that the circuit-
breaker will protect them.

Most often, transmitters that require a separate
120 volts input such as for the old-fashioned crystal
ovens, use a separate circuit and the installer
is expected to connect this entirely separate circuit
to a source of 120 volts. This was especially
important in the days of crystal ovens. You needed
to shut down the transmitter safely for maintenance
but you needed to keep 120 volts connected for
the crystal oven. Such a circuit, if it wasn't in
a transmitter, would have to be "guarded," which
means (no exposed terminals). In the real world
of transmitters, many engineers have been
bitten by the crystal heater circuits while
performing maintenance.

The site will likely obtain its 120 volts from
a "dry-type" transformer connect across a
240 or 480-volt phase. Some high-power
installations use a wye-connected 480 circuit
from which they extract 277 volts for fluorescent
lighting. You can find all the common voltages
by dividing the highest by the square-root of 3.


--
Cheers,
Richard B. Johnson
Read about my book
http://www.LymanSchool.org


 -------------- Original message ----------------------
From: "Gary Glaenzer" <glaenzer at verizon.net>
> Hello Dave;
> 
> I don't think the NEC addresses anything inside the cabinet.
> Gary
> ----- Original Message -----
>  >
>  > I believe by the NEC code, a transformer is the only way to legally derive
>  > 120 volts once you are inside the equipment cabinet.
>  >
>  > Dave