In a message dated 96-11-22, Harold Leipold writes: >We have been hearing from sales reps that customers are telling them someone >is getting ready to require a 3750 volt surge test on some telecom relays >and other telecom products.
Harold: This might be a misunderstanding of the sort that often occurs with info via the sales channel. Back in the days when IEC 380 applied, the dielectric strength test for reinforced insulation was 3750 volts rms. IEC 950, which superseded IEC 380, reduced the test voltage for reinforced insulation to 3000 volts rms. However, the 3750 volt number still persists in the minds of many people. To this day, I still run into people who are convinced that the dielectric strength test for reinforced insulation is 3750 volts. Furthermore, as you probably know, reinforced insulation is not usually required for telecom relays. Until recently, clause 6.3.3 of IEC 950 called out supplementary insulation from earth ground. This level of insulation calls for a dielectric strength of 1500 volts rms (plus additional requirements on creepage and clearance). Under the recently adopted Amendment 4 to IEC 950, the requirement for supplementary insulation has been dropped. What remains is the requirement for basic insulation in clause 6.2.1.2. By my interpretation, this requires a dielectric strength of 1000 volts rms for applications involving a standard (TNV-3) analog phone line. Of course, there is also clause 6.4, which requires a dielectric strength of 1000 volts rms for most applications. Either way, you end up with a 1000 volt requirement. So, it would appear that the overall trend in dielectric strength requirements is down, rather than up. For surge testing (as opposed to dielectric strength testing), most countries that have such requirements use either 1000 volts or 1500 volts. South Africa is a notable exception, with a 4000 volt surge. Bellcore, as you mentioned, has a 2500 volt surge that the equipment must survive. In summary, I am not aware of any new regulatory requirement to withstand a 3750 volt "surge". The 3750 volt number sounds suspiciously like the old dielectric strength test for reinforced insulation under IEC 380. Aside from the 4000 volt surge in South Africa, the Bellcore 2500 volt surge represents the worst case that I am aware of. However, for high reliability applications of residential CPE, I recommend to my clients that they use relays which can withstand 4000 to 5000 volt surges. This is because the primary protector is either missing or malfunctioning at a small percentage of residential installations. Commercial installations tend to be better maintained. So, unless you have encountered an internal purchasing specification from one of my clients, I would be surprised if the 3750 volt "surge" requirement is real. But, then again, I have been surprised before! Please let me know if you find out that this "new requirement" is real. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
