Ray & Others I am helping several engineering groups resolve EMC & LV directive requirements here at Cutler-Hammer. Some of the products are being evaluated/tested against EN61010 & UL3111 (equivalents). Some of the existing products initially designed for the US market have MOV surge Suppressers on the front end. Presently we have MOVs Line to line and each line to ground. These are wide ranging units rated 48 to 280 VAC for the power source. Our applications are in power distribution gear where we are often very close to the source of the surge. I have several comments/concerns/questions relative to your needs and for some of my own:
So far I haven't seen any obvious restrictions on using MOVs relative to EN61010/UL3111. As per the CE mark and LVD I understood that harmonized standards have precedence over national standards. All the same I have seen several examples of country safety standards that still have marketing importance. For example I understand that VDE and TUV have significant restrictions on the use of MOVs in power port connections. How are others addressing this issue? Nordic EN60950 - Although there is some parity between EN60950 and EN61010. I have found EN60950 to be more restrictive overall. Are you not sure which standard your product will be required to meet ? Couldn't you just stick with EN61010? Effectiveness of a line to line MOV when there is no grounded neutral - Ray I assume from your comments that you consider surge as primarily a common mode event. In applying the IEC 1000-4-5 test there are both common mode and normal mode (asymmetric) exposures required. Wouldn't the Line to line clamping still have value on the line to line asymmetric surges. Application of MOVs - One of my concerns with MOVs is the possible failure from too much power dissipation. My experience is that the MOV can fuse to a short and then blow open, a messy condition. We have been careful to minimize this possibility however in case of this type of failure I have been researching what can be done. Some ceramic PTC type fuses appear to limit the exposure however I understand they too can fail under surge and typically they are not rated for 230 V applications. Transient suppression without MOVs - What are the alternatives to MOVs? I have considered and seen examples using inductors in each line. What impedances values have engineers found effective to the 5 micro second surges (2-4KV ) of IEC 1000-4-5? I have seen several applications with VDE 565-2 recognized inductors apparently designed for this application. I am looking for sources that would be available in the US. In addition to the inductors I assume that some additional filtering/clamping elements would be needed - any recomendations for an overall approach? Ray As you can see I have some similar concerns. I know I haven't answered your questions but by stirring the pot perhaps we can get some discussion going. Chris Wells Sr Des. Eng. Cutler-Hammer [email protected] ------------- Original Text From: C=US/A=INTERNET/DDA=ID/Ray(u)Russell(a)leco.com, on 5/13/97 6:42 PM: Dear Friends, It seems that surge suppressers have been a topic of much discussion lately. Must be all of the surge testing to EN61000 4-5! Well at least that's what brought up our issue. We have a piece of 230V equipment that failed the surge test, (safety testing will be to EN61010). In reviewing how to install varistors, we have run into some concerns. In Europe the 230V supply is 1 line, 1 neutral, and 1 ground wire. A varistor connected between line and neutral will suppress a surge, as long as the neutral is grounded. For the 230V US voltage, there is 2 hot lines and a ground. To make the varistor effective, it seems that the varistor needs to be connected to ground. However, I know of a Nordic deviation out of EN60950 that states: (DK, FI, NO, SE). Transient protection components shall be installed in such a way that insulation for protection against electric shock will not be bridged. This means that transient protection components must not be connected to protective earthed parts in pluggable equipment or to other accessible parts. I think the concern here is the possibility of not having a good ground, and then if the over voltage is shunted to ground, the chassis could become hot. Is this a valid concern? It sounds like a "double fault" scenario to me. If so, does anyone have an alternative solution? Thank you for your consideration. Happy is the man that finds wisdom and gets understanding! Ray Russell [email protected]
