This document describes common-mode and differential-mode surge conditions. Examples of surge mitigation by non-linear limiting and linear attenuation are given.
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This document describes common-mode and differential-mode surge conditions. Examples of surge mitigation by non-linear limiting and linear attenuation are given.
2. Definitions
branch [B12]: subset of a network, considered as a two-terminal circuit, consisting of a circuit element or a combination of circuit elements
common-mode surge [B1]: surge appearing equally on all conductors of a group at a given location
NOTE 1 — The reference point for common-mode surge voltage measurement can be a chassis terminal, or a local earth/ground point.
NOTE 2 — Also known as longitudinal surge or asymmetrical surge.
differential-mode surge [B1]: surge occurring between any two conductors or two groups of conductors at a given location
NOTE 1 — The surge source maybe be floating, without a reference point or connected to reference point, such as a chassis terminal, or a local earth/ground point.
NOTE 2 — Also known as metallic surge or transverse surge or symmetrical surge or normal surge.
equipotential bonding [B11]: provision of electric connections between conductive parts, intended to achieve equipotentiality
functional-equipotential-bonding [B11]: equipotential bonding for operational reasons other than safety
node [B10]: endpoint of an electrical network branch or the junction of two or more branches
3. Surge modes
3.1 Common-mode surge
Reference [B1] defines common-mode surge as a surge appearing equally on all conductors of a group at a given location. The definition notes that the reference point for common-mode surge voltage measurement can be a chassis terminal, or a local earth/ground point and that common-
mode surge is also known as longitudinal surge or asymmetrical surge. Figure 1 shows a common-mode surge situation. In Figure 1 horizontal conductors contained in the dotted loop labeled “equipotential conductor group” all have the same surge potential.
Figure 1 —Common-mode voltage surge
The common-mode surge voltage exists between a conductor group of two or more conductors having the same surge voltage and a local reference potential node. In the case of a lightning stroke it acts as a current source at the strike point, this is why network arresters tend to be tested with current source surge generators. As the lightning surge moves through a network its waveform is often modified by flashover, surge protection operation and the network’s propagation parameters. As a result at SPD or equipment ports the surge is normally classified by peak voltage, prospective short-circuit current and their wave shapes. Surge mitigation at equipment ports typically focusses on shunt voltage limitation or attenuation being applied to the equipotential conductor group.
The level of common-mode surge current depends on the presented load of the port. Current surge propagation mitigation can be the result of voltage surge mitigation diverting the current or specific series current limiters or attenuators.
3.2 Differential-mode surge
Reference [B1] defines differential-mode surge as a surge occurring between any two conductors or two groups of conductors at a given location. The definition notes that the surge source maybe be floating, without a reference point or connected to reference point, such as a chassis terminal, or a local earth/ground point and that differential-mode surge is also known as metallic surge or transverse surge or symmetrical surge or normal surge. Figure 2 shows two differential-mode surge situations.
Figure 2 —Differential-mode surge: a) balanced and b) unbalanced
The differential-mode surge voltage exists between conductor groups, each group having a different surge voltage and consisting of one or more conductors.
Power service systems can have differential-mode surge voltages between three phases, neutral and the earth reference node.
Ethernet services can have two types of differential-mode surge. The first is the differential mode surge between the conductors of a signal twisted pair and there can be up to four twisted pairs typically in an Ethernet cable. The second type of differential-mode surge occurs in Power over Ethernet (PoE) systems, which uses a DC voltage supply to power remote equipment via pairs of twisted pair conductors. For the highest power systems all four twisted pairs (eight conductors) are used to power remote equipment. Figure 3 shows the described situation with all signal pairs and powering pairs being used.
Balanced or floating differential-mode voltage surges have a net surge of zero with respect to the reference node. For example, if the differential-mode voltage surge was 100 V peak, one conductor group would have a surge of +50 V peak and the other conductor group would have a surge of -50 V peak, making the net surge to the reference node zero; see Figure 2 a). Balanced differential-mode voltage surges are commonly produced by centre-tapped or floating secondary windings of transformers whose primary winding is subjected to an unbalanced differential-mode voltage surge.
3.2.2 Unbalanced differential-mode surge
Unbalanced differential-mode voltage surges have a low impedance connection between one of the equipotential conductor groups and the local reference potential node; see Figure 2 b). In Figure 2 b) the linkage shown from A to B connects the equipotential conductor group 2 conductors to the local reference node. In this case there is little different in the surge voltage between the equipotential conductor groups 1 and 2 and the surge voltage between equipotential conductor group 1 and the reference node. The connecting link can be permanent or only present for the surge duration due to the operation of a shunt voltage limiting. Unbalanced differential-mode voltage surges are commonly produced from common-mode surges by the asynchronous operation of shunt voltage limiters to the reference node. Reference [B1] uses the term common mode conversion to define the process by which a differential mode electrical signal is produced in response to a common mode electrical signal.
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PoE mode Apowering feed
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PoE mode Bpowering feed
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Figure 4 shows some possible generic options to mitigate common-mode surge voltages and currents. This example considers only the two conductor common-mode surge situation.
Figure 4 —Example options, a through e, for common-mode surge mitigation
Figure 4, option a) shows voltage limiting being applied from each conductor to the functional equipotential bonding to restrict the maximum surge voltage amplitude. The illustration shows the use of clamping type metal-oxide varistor (MOV) surge protective components (SPCs) to divert the common-mode surge current to the functional bonding, but actual voltage limiting technology used depends on the system. AC power systems would typically use voltage clamping type MOV SPCs, although, in some cases silicon PN-junction clamping diodes are also used. Power distribution systems often have direct lightning strikes and the MOV arresters used will often have addition air gap bypass protection. Communications systems use voltage clamping or voltage switching or both SPCs depending on the service type. Clamping technologies used are MOV, multilayer varistor (MLV) for low voltages and PN-junction clamping diode. Switching technologies used are gas discharge tube (GDT) for very high surge currents and lowest capacitance and thyristor. As well as being voltage triggered, gated thyristors can also be current triggered, which is useful for conduction coordination between SPCs. Reference [B1] contains more information on voltage limiting technologies, for specific technologies see [B4], [B5], [B6] or [B7].
Figure 4, option b) shows an in-line isolating transformer that blocks the common-mode voltage surge propagation. The transformer impulse withstand voltage must be higher than the predicted level of common-made surge voltage and the inter-winding capacitance must be sufficiently low for the capacitive feed-through impulse current not to be a problem. Reference
Figure 4, option c) shows a frequency selective filter circuit to attenuate the impulse frequencies. This technique can only be used when the service frequency band does not appreciably overlap the impulse or disturbing frequency spectrum. A common example is mains equipment power receptacles that incorporate filters to attenuate the switching mode power supply switching noise back into the supply. Reference [B1] contains more information on filter types.
Figure 4, option d) shows a common-mode choke that presents high impedance to the common-mode current surge. To work, the terminating load must be relatively low impedance and have some connection to the functional bonding, for more information see [B1].
Figure 4, option e) shows a series current limiter which limits the common-mode surge current. The current limiting technology used depends on the system. The illustration shows a constant current source type electronic current limiter (ECL), one in series with each conductor. Generally ECLs will need some form of preceding voltage limiter to prevent the blocked surge current causing voltages beyond the ECL capability. Power fault conditions are often mitigated by positive temperature coefficient (PTC) thermistors. These thermally operated current limiters are available in ceramic or polymer PTC thermistor technologies. Reference [B2] contains more information on resettable solid state current limiters. Fuses can be used to provide non-resettable current limiters.
4.2 Differential-mode surge mitigation
Figure 5 shows some possible generic options to mitigate differential-mode surge voltages and currents. This example considers only the two conductor differential-mode surge situation.
Figure 5 — Example options, a through d, for differential-mode surge mitigation
Figure 5, option a, shows voltage limiting being applied between the conductors to restrict the maximum surge voltage amplitude. The various voltage limiting technology options are the same as are given in 4.1.
Figure 5, option b, shows an in-line isolating transformer. The differential mode surge performance of an isolation transformer strongly depends on the application and transformer design.
A power frequency transformer can increase the severity of the voltage surge. As the transformer impedance at lightning frequencies will be higher the mains cabling the incident surge will be reflected back. In travelling back, the reflected surge voltage adds to the incoming surge and in extreme circumstances can double the original surge voltage, see [B8]. Voltage limiters between the conductors are usually added to prevent excessive surge voltages due to reflection.
A perfect signal transformer would not mitigate the surge and would simply transform the primary winding surge voltage to the secondary winding.
A signal transformer with a core that saturates under surge conditions can reduce the secondary circuit surge stress. When the core saturates the transformer would loses its transformation properties, effectively truncating the secondary surge duration. In the case of Ethernet transformers the typical truncated secondary surge duration is under 5 µs. see [B9]
Figure 5, option c, shows an in-line frequency selective filter circuit to attenuate the impulse frequencies. This technique can only be used when the service frequency band does not appreciably overlap the impulse or disturbing frequency spectrum.
Figure 5, option d, shows a series current limiter which limits the differential-mode surge current. Series current limiters are bidirectional and operate correctly for common-mode currents where the instantaneous currents are in the same direction in both current limiters and differential mode currents where the currents are in opposite directions. The various current limiting technology options are the same as given in 4.1.
The previous Figure 4, option d) of a common-mode choke is not shown as it does not mitigate differential surges. Under common-mode surge current conditions the common-mode choke presents a high series impedance, but due to the winding arrangement, it presents very low series impedance to differential-mode surge currents.
4.3 Combined common-mode and differential-mode surge mitigation
Protection solutions that provide both common-mode and differential mode voltage limiting invoke the concepts of direct and indirect modes of protection. A direct mode of protection is when the voltage limiting is applied directly between the surge nodes; conductor to functional bonding (common-mode surge) or conductor to conductor (differential-mode surge). An indirect mode is a circuit configuration where the protection mode path is via another node, see Figure 6 and Table 1. Indirect protection modes may not be as effective as direct protection modes because the protection level can be influenced by the other node voltage and that the protection level is the sum of the limiting voltage to that node and from that node.
Figure 6 —Examples of direct and indirect modes of voltage limiting protection
Figure 6 shows examples of voltage limiter arrangements and their protection modes for common-mode and differential mode surges on conductors C1 and C2. Figure 6 shows MOV voltage limiters just for example, but as in Figure 4, option a) the actual voltage limiting SPC technology used is not just limited to MOV.
Table 1 is a protection mode comparison for the Figure 6 circuit examples.
CONFIGURATION C1 to BONDING C2 to BONDING C1 to C2
6 a VR2, direct VR1, direct VR3, direct
6 b VR2, direct VR1, direct VR1-bonding-VR2, indirect
6 c VR1-C2-VR2, indirect VR1, direct VR3, direct
6d VR3 to VR1, direct See NOTE
VR2 to VR1, direct See NOTE
VR3 to VR2, direct See NOTE
NOTE—Assumes all the MOVs have the same clamping voltage, are symmetrical and VR1 has twice the current capability of VR2 and VR3.
Table 1 is a limiting voltage comparison for the Figure 6 circuit examples, assuming an SPC limiting voltage of 100 V for examples a, b, and c. while example d uses SPCs with a limiting voltage of 50 V.
Table 2 —Table 1 with example limiting voltages
SURGE MODE COMMON COMMON DIFFERENTIAL
CONFIGURATION C1 to BONDING C2 to BONDING C1 to C2
6 a 100 V 100V 100 V
6 b 100 V 100 V 200 V
6 c 200 V 100 V 100 V
6d 100 V 100 V 100 V
Broadband signal applications often need low capacitance protection and this is often achieved by using a bridged circuit arrangement as shown in Figure 7. Figure 7 shows an MOV voltage limiter, VR1, just for example, but as described in Figure 4, option a) the actual voltage limiting SPC technology used is not just limited to MOV. The SPC (VR1) series diodes to the nodes reduce the effective protection capacitance.
A positive common-mode surge on conductor C1 has a limiting current path of D2, VR1 and D5. The negative polarity path is D1, VR1 and D6. A positive differential-mode surge on conductors C1 to C2 has a limiting current path of D2, VR1 and D3. The negative polarity path is D1, VR1 and D4.
Reference to these resources is made for informational use only.
[B1] ITU-T Recommendation K.96: 2014 Surge protective components: Overview of surge mitigation functions and technologies
[B2] ITU-T Recommendation K.82: 05/2010 Characteristics and ratings of solid-state, self-restoring overcurrent protectors for the protection of telecommunications installations
[B3] ITU-T Recommendation K.95: 06/2016 Surge parameters of isolating transformers used in telecommunication devices and equipment
[B4] ITU-T Recommendation K.12: 05/2010 Characteristics of gas discharge tubes for the protection of telecommunications installations
[B5] ITU-T Recommendation K.77: 01/2009 Characteristics of metal oxide varistors for the protection of telecommunication installations
