Why does AC surge arrestors uses a sparkgap for neutral line rather than MOVs - Page 1
Forum section: electronics / projects

author: drakejest
author_rank: Regular Contributor
timestamp: July 29, 2025, 04:18:57 pm
content: Hello have atthese catalogfor surge arrestor for AC system from chint. I have noticed that for electrical systems that uses a Neutral line, the surge arrestor uses a Sparkgap instead of the usual MOV. Why is that? The system type where i will install this on is still not yet clear, so i guess ill just order both ?

author: Ian.M
author_rank: Super Contributor
timestamp: July 29, 2025, 05:17:17 pm
content: Enlarge the catalog and you'll see the spark gaps (where present) are between Neutral and PE (ground). A spark gap remains hi-Z below its breakdown voltage (unless its totally failed due to a severe overload plating its whole interior with metal vaporised from the electrode), whereas a MOV tends to degrade due to frequent or repetitive transients, resulting in its resistance decreasing. This would cause a low N to PE resistance fault and possibly nuisance RCD (GFCI) tripping long before the same low resistance would cause excessive current and thus dissipation if the same MOV was used in a L to N application.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 29, 2025, 05:50:10 pm
content: From memory: MOV's and Sparkgaps solve pretty much the same problem in the same way (I.e by becoming conductive in an overvoltage situation), but MOV's have a pretty limited life, and they degrade with each activation, and they fail short circuited, so they should always be behind a fuse.I think sparkgaps are more reliable and durable (but I have not checked the pulse energy they can dissipate). I think sparkgaps also have a bigger variation in their activation voltage.And what happens when a sparkgap fails? Does it just become an open circuit? That would remove the protection without any indication (except a probably black sparkgap).For a long time I've sort of disliked MOV's because of their limited activation cycles, but when combined with a fuse this is not really a problem, and only now I realize it can be seen as an advantage too. When the MOV fails, it protects your gadget by shorting it out. Just replace both the fuse and the MOV.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 29, 2025, 07:33:44 pm
content: When I worked for a TelCo, many years ago, they used spark gaps because they were simple and reliable and if they failed they failed open. MOVs fail shorted would most likely shut down down at least part of their network and someone might have have had to travel many miles to get to where the MOV was located in order to replace it and to get that part of the system back in operation again. In those days the Telcos were heavily regulated and every minute of downtime was reported to the regulatory agencies and was counted against the Telcos.

author: Ian.M
author_rank: Super Contributor
timestamp: July 29, 2025, 08:38:37 pm
content: Yes, its a pretty narrow energy and pulse duration range to ablate the spark gap electrodes *without* rupturing the casing, so *usually* a spark gap fails open

author: drakejest
author_rank: Regular Contributor
timestamp: July 29, 2025, 08:43:11 pm
content: Quote from: Ian.M on July 29, 2025, 05:17:17 pmi see, so spark gaps are more reliable.. but if thats the case why not just make the whole thing into spark gaps? why just the neutral line rather than L1 (L2andL3 for 3phase also). why does 2P,3P, and 4P (without Neutral) not have any spark gaps?

author: Gyro
author_rank: Super Contributor
timestamp: July 29, 2025, 09:13:39 pm
content: Spark gap tubes go arc discharge mode when triggered - where the arc voltage drops to a very low level (maybe 40V or less) until the current is interrupted or they blow open (are destroyed). Mains voltage and fault current wouldn't allow the arc to cease (even with zero crossings) so something (hopefully the main fuse) would need to blow. You would need an MOV in series with the spark gap to open the circuit when the transient has passed, rather defeating the object.Spark gap tubes work well in telecoms type applications because there isn't enough current (or voltage, ie. a high impedance circuit) available to sustain the arc after the transient has passed.P.S. A spark gap tube works for N-E because they should normally, apart from a fast transient maybe, be at the same potential.