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Determining the Peak Surge Current
5
SLUA818–February 2017
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Lightning Surge Considerations for PoE Power Sourcing Equipment Devices
As seen in Figure 3, eight 200-Ω resistors are used along with eight coupling elements, typically gas
arrestors, to couple the surge from the generator output to each line of the eight-wire cable
simultaneously. The 200-Ω value of the resistors is compliant with the 250-Ω maximum defined in the IEC
61000-4-5:2005 standard. The parallel combination of the eight 200 Ω resistors provides an equivalent
resistance of 25 Ω which would be in series with the internal 15 Ω resistance associated with the surge
generator. This total resistance satisfies the 40-Ω effective output impedance and peak short-circuit
requirements discussed in Section 2.
The IEC 61000-4-5:2005 standard also specifies a 1.2/50-µs generator for use in performing lightning
surge tests on power lines and short-distance signal applications. Some PSE applications may fall under
the short-distance category where the tests are performed using the 1.2/50-µs generator instead of the
10/700-µs to 5/320-µs combination wave generator. The 1.2/50-µs generator is commonly referred to as a
1.2/50-µs to 8/20-µs combination wave generator, because it is a short-circuit current waveform has an 8-
µs virtual front time and a 20-µs virtual time to half value.
Another physical consideration of PoE is discussed on page seven of theElectrical Transient Immunity for
Power-Over-Ethernet[2] application report:
On a twisted-pairs cable, the two wires of each pair are twisted together, but there is no twisting between
pairs at all (in fact, each pair is well separated from its neighbors). Consequently, a differential-mode
transient between P and N is likely with that type of cable, and this tandem of pairs can be considered as
an unbalanced line as far as the test voltages are concerned.
The implication is that the unbalanced circuit and lines category is also applicable for PoE as is the 1.2/50-
µs combination wave generator. Under these assumptions, Table 2 outlines several test scenarios using
both an eight-wire and four-wire coupling-decoupling network (CDN).
Table 2. PoE PSE Test Scenarios
TEST CONDITION EIGHT-LINE CDN FOUR-LINE CDN
Common mode (line-earth) 8-wires to earth 4-wires to earth
Single wire differential 1-wire shorted to earth with surge applied
to 7-wires
1-wire shorted to earth with surge applied
to 3-wires
Single pair differential 1-PoE pair shorted to earth with surge
applied to other 3-pairs
1-PoE pair shorted to earth with surge
applied to other pair
3 Determining the Peak Surge Current
The peak current per line needs to be considered when selecting the surge current rating of components
within the PSE, including any surge protection devices (SPDs). The path of the surge must be understood
to determine whether the component will see the current associated with one line or an additive current
associated with multiple lines. Additionally, there could be other worst case scenarios such as when only
one line or one twisted pair is subjected to the surge with the other lines open. It should be noted, that the
surge test is not normally performed with some of the lines open; rather the differential surges will be
applied as described in Table 2. Other examples include extending the surge voltage to levels above 4kV
and testing only four lines instead of eight.
Table 3 and Table 4 show the per-wire currents for both the 10/700-µs and 1.2/50-µs waveforms. The
green columns are for four-wire and the orange for eight-wire conditions. Generally speaking,
configuration one yields higher per-wire currents than configuration two and the four-wire test generates
higher currents than the eight-wire test. In most cases too, the 10/700-µs waveform yields higher peak
current with the exception of the eight-wire test in configuration two due to the maximum per-line series
resistance of 250 Ω.