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Mutual Drainage Reactors (MDRs)

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    P12776 Mutual Drainage Reactor
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    P30043 Isolation Transformer

The Mutual Drainage Reactor (MDR) is used to reduce voltage stress from conductor-to-conductor and conductor-to-shield. They are also useful in forcing balanced protector gap operation.

SNC’s MDR is a well balanced, two winding inductor. It provides low impedance mutual drainage of longitudinal voltages on telecommunications or signal pairs while maintaining a high bridging impedance to the transverse signal. (The protector shown in the picture is added by the customer.) Commonly available protectors are: Reliable Electric R123 and Joslyn Electronics 125EW, 130EW, 356Ew.)

When combined with an appropriate protection device, such as carbon air gaps or gas tubes, the MDR provided “balanced” protection on the pairs that it is treating. When used in this way, the circuit will function even during operation of the protection device.

The SNC MDR is equivalent to the 2232A design. A nickel-steel laminated core gives it a higher bridging impedance than SNC’s standard isolation transformer, which can also serve as an MDR by grounding the center tap. Both meet or exceed IEEE criteria standard 487-1980.


Length – 4.12” (10.46 cm);

Width – 3.25” (8.25 cm); Height – 3.62” (9.2 cm)

Isolation Transformer

SNC’s High Voltage Isolation Transformer is used to prevent damaging voltages or currents from flowing in communication wire pairs. Metallic DC continuity is however, not maintained between the two ends of the circuit.

The unit is a simple 1:1 600 ohm transformer that passesaudio frequency signals with a minimum attenuation. The primary winding is isolated from the secondary winding with enough dielectric material to prevent (within its rating) lightning, GPR and induced voltages from entering the communications cable. Saturation is not a problem because the magnetic core is energized by the AC signal being transmitted, not by the fault voltage.

An added benefit of the unit is that it works in the reverse direction as well…effectively isolating voltages from either line or equipment sides of the circuit. Any potential rise in the telephone cable caused by lightning or induced load and fault currents from adjacent power lines will be isolated from the substation and will not find an equalizing path to the substation wiring, equipment or grounding system.

For each pair being protected, one isolation transformer is needed. SNC’s isolation transformer is designed to exceed the expected GPR in most situations and is a 20 KV unit, rated continuous RMS voltage, 50/60Hz from line side to equipment side and to ground.


Part NumberP12776P30043
TypeMDRIsol. Transformer
DC Dielectric Test – kV (1 min. Windings to case/mtg.)2.052.3
AC Dielectric Test – kV (1 min. Windings to case/mtg.)N/A37
Min. Surge Drainage Capacity - A2 sec (>20A for 1 sec.)400400
Min. Bil. Rating – kV (Windings to case/mtg. 1.2 x 50m sec test wave)5.066
Min. Longitudinal to Metallic Balance – dB (300 – 3400 Hz)8080
Continuous Drainage Capacity – amps rms0.5N/A
Min. Bridging Impedance – Ohms (@ 100 V rms 60 Hz)100000N/A
Max. Bridging Loss – dB (20 Hz – 20 kHz)0.1N/A

Relay Rack Mounted MDRs

23” Rack – Six MDR PackageP31145