Is MOV AC Mains Overvoltage Protection to PE Still a Viable Option?

This article is a follow-up to the article entitled “New Requirements for MOVs Used for Surge Suppression on AC Mains Ports,” written by my good friend Joseph Randolph and published in the October 2018 issue of  In Compliance Magazine.

In this article, we will focus in detail on the IEC 62368-1 component requirements for metal oxide varistors (MOVs) and gas discharge tubes (GDTs) when they are used to protect pluggable Type A equipment mains ports (see Figure 1). IEC 62368-1 classifies pluggable Type A equipment as having, an having, unreliable earth/ground bonding unless special direct connections are made to the local protective earth/ground (PE). To safeguard against the failure of any mains port MOVs connected between the AC mains and PE, a GDT must be connected in series with those MOVs.

Another hazard is the possibility that a failing MOV becomes a potential ignition source (PIS). Many of the AC tests in Clause G.8 of IEC 62368-1 are intended to overload the MOV to cause failure in order to determine if the failure is non-hazardous.

AC Mains Values

Generally, equipment designs are for 120 V or 230 V nominal AC mains, or for a range like 100 V ac to 250 V ac. The equipment AC mains rating has a direct influence on the MOV and GDT voltage requirements. IEC 62368-1 Table I.1 – Overvoltage categories, assigns and overvoltage category of II for pluggable information technology equipment (ITE) connected to the building wiring. Table 12 – Mains transient voltages shows that overvoltage category II has a transient peak voltage level of 1.5 kV for 120 V ac, and 2.5 kV for 230 V ac. These values are needed for the GDT selection.

GDT Basic Insulation Requirements

One might reasonably believe that the GDT insulation requirement would be to not sparkover at the peak of the highest expected AC mains voltage, but that would be incorrect. The GDT has to provide basic insulation by meeting the requirements of the electric strength test, as presented in Clauses 5.4.9.1 solid insulation, 5.4.2 external clearance and 5.4.3 creepage distance. GDTs will not normally have problems with meeting clearance and creepage requirements but meeting the solid insulation requirements has consequences that don’t appear to have been anticipated when the standard was developed.

Clause 5.4.9.1, solid insulation, is a three-way choice. One has to select the appropriate test voltage values from Table 25 transient voltages, Table 26 peak of the working voltages and recurring peak voltages and Table 27 temporary overvoltages. The highest peak value of all three voltages becomes the GDT test voltage, which is applied as either a peak AC or a DC voltage. For example, 120 V AC mains results in test voltage values of 1.5 kV peak (overvoltage category II), 243 V peak and 2 kV peak. Similarly, 230 V AC mains gives test voltage values of 2.5 kV peak (overvoltage category II), 465 V peak and 2.5 kV peak. Table 27 has the highest voltage test value of 2 kV dc or 1.41 kV ac for 120 V ac and Tables 25 and 27 are equal highest at 2.5 kV dc or 1.77 kV ac for 230 V ac.

There can be no insulation breakdown (also known as sparkover for GDTs) during the application of the test voltage. Changing the minimum GDT sparkover voltage set by the test voltage to a nominal sparkover voltage values gives a 2.5 kV GDT for 120 V ac and 3 kV for 230 V ac. Adding these GDTs in series with MOVs that are connected between AC mains and PE means that there can be no conduction or protection at voltage levels below 1.41 kV ac or 1.77 kV ac, depending on AC mains voltage. As a consequence, designers may be forced to considerably increase the inherent insulation withstand voltage in the equipment, since it must be higher than these AC/DC test conditions, and since a 3 kV GDT might sparkover at 4 kV for impulse voltages. Taking this approach would facilitate conducting post manufacture hi-pot testing. As the overvoltage protection level to PE is now much higher, some designers may dispense with the MOV protection to PE and just beef up the equipment inherent insulation withstand voltage instead.

Normative References

Documents that are referred to in part or whole in IEC 62368-1 and are listed as normative constitute requirements of the standard. Ideally, one would have copies of all the normative documents to fully implement the requirements standard. Problems arise when the requirements of normative documents conflict with each other, with the text of the standard itself, or both. In the standard, there are conflicts arising from normative references IEC 61051-2/AMD1, Varistors for use in electronic equipment – Part 2: Sectional specification for surge suppression varistors, and IEC 61643-331, Components for low-voltage surge protective devices – Part 331: Performance requirements and test methods for metal oxide varistors (MOV). To resolve this situation, an interpretation is required as discussed in the following sections. (Note that, unless an interpretation comes from the IEC group that created the standard, you must assess for yourself the applicability of any interpretation to your own situation.)

MOV or Series Connected MOV and GDT Requirements

Clause G.8, Varistors, has four subclauses that contain the MOV requirements and these are examined in the following sections. Some subclauses only apply only to the MOV while other subclauses apply to the series connected MOV and GDT as well.

General Requirements (G.8.1)

This clause offers a choice between the selected MOV climatic categories from two different normative standards, IEC 61051-2 or IEC 61643-331. Alternative values are in both standards and these could be used because the standards are normative. The selected IEC 61051-2 climatic values duplicate those found in Annex Q, Voltage dependent resistors (VDRs), of IEC 60950-1.

On maximum continuous operating voltage (MCOV), the offered choice is confusing. MCOV must be at least 1.25 times the equipment rated voltage, or at least 1.25 times the upper voltage of the equipment rated voltage range. For equipment nominally rated at 230 V ac, the minimum MCOV is 288 V ac. But, for an equipment rated voltage range of 100 V ac to 250 V ac, the minimum MCOV is 313 V ac. Here and in other places, this penalizes range-rated equipment. The problem is that the reference voltage can have two different values. A greater consistency between these two approaches could be achieved by increasing the nominal 230 V ac value by 10% (the maximum 230 V ac limit in many European countries), and then multiplying the 1.1 times nominal rating or the upper voltage of the rated voltage range by 1.14 times. Annex Q of IEC 60950-1 used a lower multiplication factor of 1.2.

The surge current capability of the MOV is verified by applying 10 positive surges or 10 negative surges using a 1.2/50-8/20 combination wave generator (CWG). Table 12 – Mains transient voltages, is referenced for the selection of voltage, but this is countermanded by the statement that any AC mains under 300 V ac is regarded as mains at 300 V ac. This means that both 120 V ac and 230 V ac have the same overvoltage category II voltage of 2.5 kV.

The normative IEC 61051-2 has a cut-down version of Table 12 having AC mains voltages of up to 300 V ac and up to 600 V ac with overvoltage categories I to III. For overvoltage category II and up to 300 V ac, the CWG settings are 2.5 kV/ 1.25 kA. This appears to define the test values, but the following IEC 62368-1 text confuses things further by stating that, to test to Overvoltage Category IV of Table 12, the CWG settings are 6 kV/ 3 kA. The term “shall” is not used here, which would have made it a mandatory requirement, so what does it mean? Possibly, this statement is grandfathering the values presented in Annex Q of IEC 60950-1, which requires a CWG test level of 6 kV/3 kA (2.4 times higher than the overvoltage category II level). Alternative surge tests from IEC 61051-2 or IEC 61643-331 are allowed, which would endorse an overvoltage category II verification, if IEC 61051-2 is chosen. The remaining part of G.8.1 is concerned with component body flammability.

General safeguards against fire (G.8.2.1)

This clause emphasizes that the MOV shall be regarded as PIS and that steps should be taken to reduce the likelihood of ignition and prevent fire spread.

Varistor overload test (G.8.2.2)

This ac step stress test is applied to either an MOV or an overvoltage protective circuit containing an MOV that is connected across the AC mains (L to N), line to protective earth (L to PE), or neutral to protective earth (N to PE). The test concept is to progressively step up the power in the MOV until it fails. The pass criteria are that there shall be no hazardous events either during or after the test. The test voltage used is based on the value of V_r, which is defined as either the equipment nominal AC voltage or the upper limit of the range AC voltage.

The test circuit depicted in Figure 2 applies an AC voltage source of 2 x Vr, to the MOV or an overvoltage protective circuit via the resistor RX and test switch, SW. Resistor RX defines the prospective short-circuit current. The first level of prospective ac is 0.125 A. The test is applied until the monitored current falls to zero or temperature stability occurs. Subsequent test steps halve the Rx value to double the current for each new test, e.g., 0.25 A, 0.5 A, 1 A, 2 A, 4 A, etc. Testing is terminated when the circuit opens. Testing remarks are that during the test the operation of a current or temperature limiter will stop the current flow and that current flow may start due to the operation of a GDT. If the second remark refers to the protective circuit GDT, this cannot happen as, has been shown earlier, the required GDT voltage is over 2 kV.

There are two drawbacks with the testing as described. Testing stops when the failure event occurs, such as a disconnector operation. Thus, the disconnect mechanism is not tested at higher breaking current levels where there may be substantial operational lag time and the currents may exceed the disconnector capability. A voltage source of 2 x V_r limits the ability of the circuit to induce MOV failure. Selecting an MOV with MCOV > 2xV_r means there can never be any substantial current conduction and the test becomes inconsequential. IEC 62368-1, Table G.10 acknowledges this by stating that no testing is required if MCOV > 2xV_r. For testing a single MOV, a source voltage of 2 x MCOV would ensure the MOV could be exercised to failure. As described earlier, the protective circuit GDT sparkover voltage will prevent current flow, making the test inconsequential.

Tests like G.8.2.2 have led to the creation of the thermally protected MOV, which is a hybrid component incorporating an MOV and thermally-operated disconnect. Thermal disconnection prevents the MOV from excessively overheating and becoming a hazard. Figure 3 shows an example characteristic for a thermally-protected MOV. The thermally-protected MOV is mainly useful for connection across the AC mains and is unnecessary when the MOV is in series with a high voltage GDT.

Temporary overvoltage tests (G.8.2.3 – uses IEC 61643-11 8.3.8.1 & 8.3.8.2 test circuits)

For temporary overvoltage (TOV) testing, test circuits from the normative IEC 61643-11 standard are used. These tests are applied to a protective circuit containing an MOV connected line to protective earth (L to PE), or neutral to protective earth (N to PE). Notice there isn’t a test for the AC mains (L to N) terminals. The worldwide variation of AC mains distribution systems and voltages makes it difficult to articulate a single, universal explanation of TOV testing, so this description is limited to 120 V ac and 230 V ac supplies delivered from a single-phase TN-S distribution system.

Before starting the TOV testing, the protective circuit must be preconditioned with the CWG surge specified in G.8.1 (IEC 62368-1:2018 says G.8.2, but that’s incorrect!). However, due to the high-voltage GDT sparkover voltage requirement, the CWG 2.5 kV overvoltage category II test could be inconsequential as the GDT may not sparkover. Likewise, none of the following TOV tests will cause the protective circuit to conduct because the applied voltage is less than the GDT sparkover voltage.

IEC 61643-11, 8.3.8.1, low voltage system TOV faults, applies to TN, TT and IT systems. In Figure 4, the U_REF voltage source represents the highest expected continuous AC mains voltage and is defined in IEC 61643-11, Annex A. For 120 V ac and 230 V ac, the U_REF values are 132 V ac and 255 V ac respectively. The U_TOV voltage source represents the TOV condition defined in IEC 61643-11, Annex B, Table B.1. The TOV test times are 5 s and 120 min, with the 5 s U_TOV voltage being 1.32 × U_REF and the 120 min U_TOV voltage being 1.73 × U_REF. For 120 V ac and 230 V ac the U_TOV voltages are 174 V ac and 337 V ac for a 5 s TOV, and 229 V ac and 442 V ac for a 120 min TOV. The testing requires that the protective circuit must be capable of withstanding the 5 s TOV condition but is allowed to exhibit a non-hazardous failure for the 120 min TOV condition. Prospective short-circuit currents from the voltage sources are controlled by resistors R_REF (125 A short-circuit current) and R_TOV (MOV voltage > 0.95 U_TOV peak).

IEC 61643-11, Clause 8.3.8.2, high (medium) voltage system TOV faults, only applies to TT and IT systems as it cannot occur in TN systems. Hence, the following description is for informational purposes only. In Figure 5, the 1200 V rms voltage source represents the TOV condition defined in IEC 61643-11, Annex B. A TOV test time of 0.2 s is used with the protective circuit, connected to the PE and L or N terminals. The protective circuit can either withstand this condition or exhibit a non-hazardous failure. Basically, the Figure 5 test circuit inserts 1200 V rms TOV between the AC mains supply and PE for 0.2 s (SW1 position 2), after which the PE is connected to neutral via discharge resistor R_PE (SW1 position 3). Prospective short-circuit currents from the voltage sources are controlled by resistors R_REF (125 A short-circuit) and R₁₂₀₀ (300 A short-circuit).

Design Examples

The following examples are for pluggable equipment Type A deemed as having an unreliable PE connection, thereby necessitating series connected MOV and GDT circuits connected to PE.

Two Terminal Protection

In this example, individual two terminal protection is directly applied to L to N, L to PE and N to PE (Figure 6).

Standard MOVs can be used for MOV2a and MOV2b because these MOVs are in series with a high voltage GDT and G.8.2.2, Varistor overload, and G.8.2.3, TOV, tests are benign. MOV1 protects the L to N terminal pair. To use a standard MOV for MOV1, the MCOV needs to be higher than twice the AC mains voltage to be excluded from the G.8.2.2 test, and there isn’t any G.8.2.3 testing for the L to N terminal pair. For 230 V rms AC mains, MCOV > 500 V rms and for 120 V rms AC mains, MCOV > 265 V rms. For lower values of MCOV, the L to N terminal pair G.8.2.2 test probably means a thermally-protected MOV needs to be used.

Three Terminal Protection

In this example, shared protection is used for L to N, L to PE and N to PE (Figure 7).

The L to PE and N to PE terminals are protected by the series-connected MOV1a and GDT circuit, and the series-connected MOV1b and GDT circuit respectively. Standard MOVs are justified for the L to PE and N to PE conditions because the series connected MOV and GDT combination is excluded from G.8.2.2, Varistor overload, test.

Protection for the L to N terminal pair is given by MOV1a and MOV1b in series. In this case, assuming both MOVs are the same, the MOV MCOV needs to be higher than the AC mains voltage to be excluded from G.8.2.2, Varistor overload, test to allow standard MOVs to be used. For 230 V rms AC mains, this makes MCOV > 255 V rms, and for 120 V rms AC mains, MCOV > 132 V rms.

Single Fault Conditions

Consideration needs to be given to the possible short-circuit or disconnection of passive components. It is important to note that this does not apply to components such as the series GDT that serve as a safeguard compliance with the relevant requirements of Annex G. In Figure 7, MOV1a and MOV1b are in series across the AC supply, so what happens if one goes short-circuit and the full AC mains voltage appears across a single MOV? Accounting for this single fault condition would increase the high mains 230 V ac/120 V ac MOV MCOV values to MCOV > 255 V rms and MCOV > 132 V rms respectively. For a MOV with MCOV values in the range of 100 % to 63 % of the AC mains voltage, the L to N terminal pair G.8.2.2 test probably requires a thermally-protected MOV, unless a fire enclosure is present.

Summary

For most pluggable equipment Type A, the IEC 62368-1-mandated series GDT high voltage withstand voltage requirement removes any need for testing the protection circuit to G.8.2.2, Varistor overload, and G.8.2.3, TOV, as the tests are inconsequential. It is difficult to believe the outcomes of no electrical stress tests or increases in inherent equipment insulation withstand voltage were planned when the standard was developed. Standard MOVs can be used in the L to PE and N to PE protection circuit. The L to N protection can use a standard MOV or MOVs provided the total MOV MCOV > 2 x AC mains supply voltage or, alternatively, the use of a thermally-protected MOV for lower MCOVs.