Doug- I don't see the need for a fuse in series with a neutral-to-ground MOV. The MOV should never get to the point where it is conducting on the peaks of the normal voltage waveform to cause thermal runaway, since this voltage is so low (I×R drop in the wire due to the load).
If a fuse is still wanted or needed to cover the case of miswiring (hot and neutral reversed), a blown fuse could be monitored. Assume correct wiring. Run a rather high value resistor from the hot wire to the junction of the fuse and MOV. Then normally there will be voltage across the resistor which will disappear with a blown fuse. Or no voltage across the MOV normally, but voltage will appear with a blown fuse. Don From: [email protected] To: [email protected] Cc: "emc-pstc" <[email protected]> Date: 09/24/2012 07:55 AM Subject: Re: MOV failure detection Don, Voltage sensing the fuse is useful on any line which has enough voltage with respect to earth. The MOV tied to the neutral is much more difficult to detect in this way. However, I would venture to say that this MOV is not subjected to so many impulses as the other. The line-to-line MOV presents a new problem, if present. I have seen phase monitors used which are optocouplers using neon lamp. This can provide isolation to your monitoring circuits. Doug Douglas E Powell Independent Compliance Consultant http://www.linkedin.com/in/dougp01 http://www.linkedin.com/in/dougp01 -----Original Message----- From: [email protected] Date: Mon, 24 Sep 2012 07:23:36 To: <[email protected]> Cc: <[email protected]> Subject: Re: MOV failure detection In normal use, the way MOVs wear out is that the conduction voltage decreases as they continue to absorb the energy from voltage transients. They go over the cliff when the conduction voltage drops to the peaks of the power line voltage, at which point they are absorbing energy continuously. If nothing is done, the MOVs will get very hot and let the smoke out, quite possibly including the smoke contained in the contents of the rest of the room. The way to protect against this failure mode is to put a fuse in series with the MOV. If this is done, there are two possible indicators that can be used to signal whether transient protection is taking place. Quite often a green LED will be lit using the voltage across the MOV to indicate normal operation and transient protection. The alternative is to use a red LED powered by the voltage across the fuse. It will stay off until there is a fault that blows the fuse, and then comes on indicating loss of protection. An MOV can fail open, but most often that happens when there is a spectacular event like a lightning strike. This may blow the fuse, or possibly cause the device containing the MOV to explode, in which case there is a fairly clear failure indication of a different sort. Donald Borowski EMC Compliance Engineer Schweitzer Engineering Labs Pullman, WA, USA From: [email protected] To: [email protected] Date: 09/23/2012 08:31 AM Subject: MOV failure detection Sent by: [email protected] Aside from all the regulatory issues of leakage current and nasty failure modes of MOVs. What methods have been used to detect a failed transient suppression device in equipment? Or, is there some expectation the devices will survive a warranty period and that alone is good enough? Seems like any detection schemes would also have a series of regulatory issues and be a significant cost adder to the product. Doug Douglas E Powell Independent Compliance Consultant http://www.linkedin.com/in/dougp01 http://www.linkedin.com/in/dougp01 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
