Interrupted testing has become more common, and many conventional tap-controlled rectifiers weren’t designed to interrupt easily. Traditional configurations often require a portable interrupter and heavy temporary cabling, which can be time-consuming and can even change rectifier output during the test.
Why it matters
Interrupted surveys and testing are only as good as the conditions under which you test. If installing a portable interrupter changes the rectifier output, your results may not reflect normal operating conditions - leading to confusion in the data and extra time in the field.
The issue with conventional interruption
Most conventional rectifiers require a portable interrupter to be installed in one of two ways:
· Remove an AC secondary tap bar and install the interrupter
· Disconnect the negative DC output cable and install the interrupter in series
Either method can introduce practical problems:
· Added resistance from interrupter switch/cables can reduce DC output compared to the “before” condition
· Heavy-gauge cables and temporary connections are awkward and slow
· If the rectifier is operating above 100 DC amps, a heavy-duty interrupter may be required (often costing $1,500 - $2,500)
· In some cases, induced AC on the pipeline during interruption can influence results and create misleading “OFF” data
A better solution: convert to a “Hybrid Stack”
A practical method to enable electronic interruption on conventional rectifiers (DC output ratings up to 375 amps) is to convert the rectifier stack to a Hybrid Stack. This option has been available for decades, but it’s often overlooked because it’s typically specified only when ordering new equipment.
What changes?
A typical single-phase rectifier stack uses four diodes to convert AC from the transformer into DC. By replacing two standard diodes with Silicon Controlled Rectifiers (SCRs) and adding a small control board and relay, the rectifier can be interrupted electronically - without temporary heavy cabling or tap-bar removal.
How it works (simple explanation)
An SCR behaves like a diode - but it can be switched off. It includes a “gate” circuit:
· Apply a small gate signal and the SCR conducts like a diode
· Remove the gate current (typically ~100 mA) and, when conditions reverse, the SCR becomes an open circuit until the gate signal is restored
That means the rectifier can be turned ON/OFF for interrupted testing using a low-current control signal, rather than moving high-current cables.
What the drawing shows (see below)
· Drawing 1: Typical portable interrupter method (conventional approach)
· Drawing 2: Hybrid Stack components (two SCR diodes + control board + 12V DC relay) compatible with portable interrupters or RMUs providing a 12V DC interrupt signal
· Drawing 3: Hybrid Stack configuration for portable interrupters using conventional switch contacts/electronic switch, using a 12V AC relay powered from rectifier taps (with permanently installed wiring/protection)
Why isn’t this used more often?
Many CP designers and technicians simply don’t realize this option is available or didn’t need it at the time the rectifier was specified.
The good news: converting existing rectifiers to allow electronic interruption is a viable, practical alternative to interrupting tap bars or DC output cabling. The electronic components, including the SCRs and interface control board, necessary to perform this task are readily available from most cathodic protection rectifier manufacturers and distributors. And this method can be used on both air- and oil-cooled, single and three-phase rectifier units.
Real-world example (cost-effective upgrade)
For an interrupted pipe-to-soil survey, an existing single-phase, oil-cooled rectifier (18V, 150A, Class I Div 2) was modified with a Hybrid Stack. Component cost was under $800 (excluding labor), and the retrofit could be completed in less than four hours with the right skills/equipment, compared to replacing the rectifier at approximately $11,000.
Bottom line
If you’re doing interrupted testing more frequently or you need interruption compatibility with remote monitoring, converting a conventional rectifier to a Hybrid Stack can:
· Reduce setup time and temporary wiring
· Avoid output changes caused by interrupter cable resistance
· Enable clean electronic interruption using low-current control signals
· Extend the useful life of existing rectifiers in demanding applications
Drawing #1 - Typical portable interrupter method (conventional approach)
Drawing #2: Hybrid Stack components
(two SCR diodes + control board + 12V DC relay) compatible with portable interrupters or
RMUs providing 12V DC interrupt signal
Drawing #3 - Hybrid Stack configuration for portable interrupters using conventional switch contacts/electronic switch, using a 12V AC relay powered from rectifier taps
(with permanently installed wiring/protection)
Need help deciding if your rectifier can be converted or sourcing the SCRs/control board?
Contact Farwest Corrosion Control.
We can help evaluate the rectifier, recommend the right approach and support the retrofit plan.
Farwest Corrosion Control Company
888-532-7937 FarwestCorrosion.com
Headquarters: 12029 Regentview Avenue, Downey, CA 90241