AIR COOLED CATHODIC
PROTECTION RECTIFIERS
APPLICABLE STANDARDS:
NEMA Publication No. MR 20-1958, reaffirmed by NEMA
1971-Semiconductor
Rectifiers, Cathodic Protection Units.
NEMA Standard Publication No. 250-1979, including Rev.
No. 1 – December 1980, Enclosures for Electrical Equipment (1000 Volts
maximum)
GENERAL:
The AC input of the rectifiers shall be 115, 230 or
460 VAC – single or three phase – 60 Cycle.
ENCLOSURE:
The rectifier case shall be NEMA 3R, completely weatherproof for
outdoor use. The case shall be constructed of not less than eleven gauge steel. All
fabrication welds shall be clean and smooth. The entire case shall be hot dip galvanized
per ASTM-123. The cabinet is to be equipped with a suitable pole mounting channel or base
mounting legs welded to the case prior to galvanizing. Screens in the ventilation openings
at the top and bottom of the case shall be double zinc plated. The front door shall be
hinged on one side and have a positive locking device capable of taking a lock with a
3/8" diameter shank. In addition to the front door, the left side panel of the case
shall be hinged and easily removable. The case shall have a slide out rack assembly for
accessibility. A grounding lug to accommodate a #6 wire shall be provide on the outside of
the cabinet.
TRANSFORMER:
The transformer shall be specifically designed for use in a
cathodic protection rectifier, having separate primary and secondary copper windings. Wire
size on both windings is to be based on a minimum of 1,000 circular mils per ampere. The
material used in the core of the transformers shall be of such quality that core losses do
not exceed 0.62 watts per pound. The amount of core material shall be no less than the
amount given by the following formula:
AC = The square root of the watts in the primary divided by 5.58
AC = The minimum area of core in square inches. The core area is
figured as the cross sectional area of that portion of the core which passes through the
coil.
The transformer shall be immersed in class F transformer varnish until
all taps, insulating materials, outer wrapping and coil windings have been completely
saturated. The transformer will then be oven baked until completely dry.
When a three phase rectifier is specified, it shall be provided with a
three phase transformer with three separate wound legs or three separate transformers.
The secondary shall have a sufficient number of coarse and fine taps to
provide a minimum of 18 equal step of adjustment. These taps shall be brought out to link
bar arrangement for adjusting the output of the rectifier.
The link bars shall be terminated on at least a 5/16" stud lug
that has one end drilled out so that the transformer tap wire can be soldered to the back
of the stud. Quick change plastic knobs with brass inserts shall be used to connect the
link bars to the studs.
RECTIFYING ELEMENTS:
The rectifier stack shall consist of high current
density selenium cell arranged to give full wave rectification. Ratings
shall be within the manufacturer’s recommended current rating for
continuous operation with a 50 degrees C ambient temperature. The RMS
voltage rating of the rectifier stack shall be sufficient to withstand,
without damage, the full output of the transformer secondary when the load
is disconnected from the D. C. terminals, ie., under open circuit
conditions.
When silicon is used as the rectifying element, current rating for
continuous operation shall be for 50 degrees C. ambient and the PRV rating of the diodes
shall be at least 1200 PIV. The diodes shall be protected against high voltage surges with
selenium surge suppressors.
Current and voltage shall be de-rated for higher ambient temperature,
where required, and in accordance with the manufacturers recommendations.
PROTECTIVE DEVICES:
The entire unit is to be protected against overload and short
circuit with a fully magnetic circuit breaker of proper rating connected between the A.C.
supply and transformer primary.
Circuit breakers shall have two poles for single phase units and three
poles for three phase units. In the case of silicon rectifiers only, fuses shall be
inserted in one leg of the A.C. secondary of single phase units and in at least two of the
secondary legs of a three phase unit. All fuses greater than 35 amps shall be bolt in
style and shall not rely on pressure type fuseholders.
D.C. METERS:
One D.C. voltmeter and one D.C. ammeter shall be provided. Each
will have an accuracy of 2% full scale. Hoyt # 17/3 meters or approved equal shall have
minimum scale lengths of 1 5/8". The ammeter shall be connected to an external shunt
with and accuracy of at least 1%. The shunt shall be plainly marked to show ampere rating
and millivolt drop. This shunt is to be mounted on the front panel of the rectifier so as
to be readily accessible for meter accuracy checks.
D.C. TERMINALS:
Solderless lugs rated for full rectifier output current shall be
provided for the positive and negative output terminals of the rectifier and shall be
mounted on an insulated panel. Output terminals shall be clearly identified on the panel
as "Positive" and "Negative".
WIRING AND CONDUCTORS:
All wiring within the rectifier, except the meter circuits, shall be of
the high temperature motor lead wire with a minimum of 105 degrees C rating. Wire size
shall be based on not less than 500 circular mils per ampere. All current carrying bolts,
terminals and connections made through the panel shall be either soldered to the bolt head
or made by use of double nut method, so as not to depend on the compression strength of
the panel to maintain a tight connection. Tap changing studs and output lugs shall be a
minimum of 5/16" diameter.
RECTIFIER DATA:
Each rectifier shall be provided with an engraved metal nameplate
with the following information.
