| New requirements for Voltage Dependent Resistors under UL/ IEC 60950-1, 2nd Edition
 In December 2005, the Second Edition of IEC 60950-1 was published to supersede its predecessor, the First Edition of IEC 60950-1. In the United States and Canada, the bi-national standard CSA C22.2 No. 60950-1/ UL 60950-1, Second Edition, will be based on the Second Edition of IEC 60950-1, and like its predecessors will include U.S./ Canadian National Differences to address National considerations. The anticipated publication date of the UL/CSA Standard is Quarter 1 2007. A UL Effective Date has not been assigned yet.
Changes in requirements for surge suppression devices:
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The Second Edition of IEC 60950-1 provides a new Sub-clause, 1.5.9, Surge Suppressors, as summarized below: |
| Summary of requirement |
Rationale |
| Clause 1.5.9.1 — General |
| A surge suppressor used in a Primary Circuit now is required to be a voltage dependent resistor (VDR) type, and comply with the component requirements per Annex Q, VDRs. A VDR is defined as a varistor or metal oxide varistor (VAR). A gas tube, carbon block or semiconductor device is not a VDR. (Note: Annex Q requires a VDR to comply with specified parts of IEC 61051-2, Varistors for Use in Electronic Equipment. Any type of surge suppressor may be used in Secondary Circuit.) |
Since component requirements for surge suppressors previously were not part of IEC 60950-1, IEC TC 108, the technical committee responsible for IEC 60950-1, has established a new requirement that surge suppressors used in Primary Circuits be the VDR type, and that they comply with the specified component requirements in IEC 61051-2. |
| Clause 1.5.9.2 — Protection of VDRs |
| For protection of VDRs, overcurrent protection or a similar interrupting device/ means is required to be supplied in series with the VDRs. |
VDRs may be subjected to temporary overvoltages, thermal overload due to increased leakage current, or burning and bursting in the event of a short-circuit fault. Therefore, they require a protection device in series with the VDR in case of such events. |
| Clause 1.5.9.3 — Bridging of Functional Insulation by a VDR |
| Bridging of Functional Insulation by a VDR is permitted |
Since Functional Insulation is associated with risk of fire, and VDRs used in a Primary Circuit are contained in Fire Enclosures, the protection required by 1.5.9.2 (combined with a Fire Enclosure) has been established as a suitable level of protection of VDRs bridging Functional Insulation. |
| Clause 1.5.9.4 — Bridging of Basic Insulation by a VDR |
| Bridging of Basic Insulation by a VDR is only permitted if one side of the VDR is “reliably” earthed (e.g., Pluggable Equipment Type B). |
If VDRs provided in Primary Circuits operate, there should be a reliable path to protective earth as an additional level of protection, since Basic Insulation is being bypassed and is associated with risk of Electric Shock. |
| Clause 1.5.9.5 — Bridging of Supplementry, Double or Reinforced Insulation by a VDR |
| Bridging of Supplementary/ Double/ Reinforced insulation by a VDR is not permitted. |
Since VDRs establish a conductive path across insulation once they operate, they are not permitted to bridge Supplementary, Double or Reinforced Insulation since the associated non-SELV voltages bridging the insulation could present a risk of Electric Shock. |
The First Edition UL 60950-1, as did previous editions, already contains a mandatory component requirement in Annex P.1 that surge suppressor are required to comply with UL 1449, Transient Voltage Surge Suppressors. In the pending Second Edition of UL 60950-1, its Annex P.2 now will
allow a VDR used in a Primary Circuit to comply with either IEC 61051-2 or UL 1449. Compliance with UL 1449 still will be required for non-VDR surge suppressors used in other permitted applications and when considered safety critical components.
Impact on ITE
For the first time IEC 60950-1 explicitly addresses the general use of surge suppression devices in ITE. Therefore, the practices of different manufacturers and National Certification Bodies (NCBs) will be driven to be more closely aligned. Affected industry segments will include ITE Power supplies and other ITE intended for connection to mains supply that have built in power supplies and surge suppressors in the Primary Circuit.
A key impact will be that only VDRs can be used for surge suppression in the Primary Circuits. Since other devices, such as gas discharge tubes, carbon blocks and semiconductor devices with non-linear voltage/ current characteristics are not considered VDRs, the use of such types of device in the Primary Circuit now will be restricted. There could be some impact on existing designs.
For UL certifications only, the Second Edition of UL 60950-1 will allow the option for VDRs to comply with either IEC 61051-2 or UL 1449 since the technical requirements in IEC 61051-2 are considered an adequate set of technical component requirements in the United States, as is UL 1449. However, for UL certified ITE, if a VDR complies only with IEC 61051-2, and is not also UL Recognized to UL 1449, it will also need to be subjected to UL’s component acceptance policy, which includes UL Follow-Up Services considerations.
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