Specifications

Chapter 4 – Coupling Circuits
148 PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Boo
k
Line Surge Protection
Coupling circuits that connect the PL Smart Transceiver to the power mains require the addition of one or more
components to provide protection for the PL Smart Transceiver from the high-voltage surges that occur on power
distribution systems. Primarily lightning induced, these surges can present voltages of up to 6kV at very high current
levels, for brief periods, to the coupling circuits inside buildings and homes. Even higher voltages can be seen on mains
wiring outside of buildings.
The level of surge protection required for a given product often depends on the installed location of the product to be
protected. Devices connected to branch circuits within a building or home are typically subject to the lowest level of
surge stress. Devices connected at, or close to, the power entry point of a building or home (for example, electrical
meters and main breaker panels) are subject to higher levels of surge stress. Devices connected to outdoor wiring are
subjected to the highest levels of surge stress.
Standard tests for surge immunity are defined in what is commonly called the IEEE surge “Trilogy” (IEEE C62.41.1[8],
C62.41.2[9] and C62.45[10], and CEI/IEC 61000-4-4[7]. Both documents classify levels of surge stress by the type of
surge waveform (either Ring wave or Combination wave), surge voltage, and surge current. In addition to describing
standard test methods, both documents also suggest surge immunity levels based on the application environments
described above.
The recommended test procedures described in the two sets of standards are the same, but the suggested immunity levels
called out in the IEEE Trilogy substantially exceed the suggested immunity levels of CEI/IEC 61000-4-5. The more
severe (and thus more conservative) immunity levels called out in the IEEE Trilogy were used in characterizing the
recommended surge protection circuitry shown in the PL Smart Transceiver coupling circuit examples of this chapter (up
to the limits of available test equipment).
Metal oxide varistors provide a low-cost, yet effective, way to absorb and divert most transient surge energy away from
sensitive circuitry. It is important to select a varistor with a high enough voltage rating so that the varistor will not clamp
on occasional long duration line voltage swells - which could otherwise destroy a varistor due to excessive heat.
Specifying an appropriate varistor is complicated by the fact that some manufacturers use part numbers that correspond
to the DC voltage that results in 1mA of current flow, while other manufacturers number their parts according to the
maximum allowable AC voltage. To allow for line voltage swells without damaging the varistor, a part with a DC rating
at least 35% above the peak AC line voltage is recommended (e.g., a 470VDC varistor for a 240VAC mains). For AC
rated varistors a 25% allowance above the nominal line voltage is recommended (e.g., a 300VAC varistor for a mains
voltage of 240VAC). When selecting varistors for use in 3-phase coupling circuits it is wise to also allow for the
possibility that the neutral and phase connections might inadvertently be reversed, resulting in 1.7 times the nominal
phase-to-neutral voltage across 2 of the 3 varistors. The varistors specified in the coupling circuits shown in this chapter
are all selected in accordance with the above guidelines.
Surge protection with earth-return coupling is often constrained by the need to maintain low leakage current. A varistor
connected between line and earth adds leakage current that might result in violation of applicable safety standards. For
this reason the use of a varistor for surge protection in single-phase line-to-earth coupling circuits is often prohibited.
Adequate surge immunity in single-phase branch circuit applications can be achieved without varistors by the use of an
X2-type capacitor in the C101 location. Surge immunity of single-phase line-to-earth coupling circuits can be increased
to provide protection for power entry and outside wiring applications by the use of a gas tube surge arrester between line
and earth. Varistors can be used in 3-phase earth-return applications because the leakage current from each of the phases
cancels.
If a gas tube surge arrestor or varistor is used between line and earth, it will have to be loaded on the PCB after hi-pot
testing. Hi-pot testing between line and earth is usually performed at voltages above the break-down voltage of gas tube
surge arrestors (or the clamp voltage of varistors). The hi-pot test will fail if a gas tube surge arrestor fires (or the varistor
clamps) during the testing. Note that the use of gas tube surge arrestors is not recommended in applications where
equipment will be powered from an electronic AC power source (as opposed to utility power distribution).