understanding "double pole/neutral fuse" - Page 1
Forum section: electronics / beginners

author: m4rtin
author_rank: Regular Contributor
timestamp: April 22, 2013, 08:11:21 am
content: I have one network switch which has a yellow sticker on it with "This unit may have double pole/neutral fusing" warning. What exactly does this mean? What is "double pole/neutral fuse"?

author: vk6zgo
author_rank: Super Contributor
timestamp: April 22, 2013, 08:21:30 am
content: I think they are telling you that both the Active & Neutral line have fuses in them.This is usually frowned on in 220/230V countries,but if the device originates in North America,having both legs of the 240v line fused is a requirement of their peculiar centre-tapped pole transformer form of 240V,where both legs of the supply are 120V above Neutral,& hence,Earth.I have seen,& worked on quite a few bits of equipment like this,which seem to have been passed by the Electrical authorities in Australia,despite having a feature which is supposed to be a "no-no" here.

author: m4rtin
author_rank: Regular Contributor
timestamp: April 22, 2013, 08:50:58 am
content: Quote from: vk6zgo on April 22, 2013, 08:21:30 amOk, I see. However, what is the advantage of having fuse both in active and neutral line?

author: Alana
author_rank: Frequent Contributor
timestamp: April 22, 2013, 09:03:55 am
content: Picture sometimes tells 1000 words.

author: Psi
author_rank: Super Contributor
timestamp: April 22, 2013, 09:07:08 am
content: Quote from: m4rtin on April 22, 2013, 08:50:58 amThere's only an advantage in a country which has the system vk6zgo and Alana describe above.In that system your have two active 120V connections (instead of one 230V active and one 0V neutral).In a country that uses your typical 230V active / 0V neutral system there's a big safety issue with fusing neutral.If the neural fuse blows first then 230V remains present on the faulty device through the other fuse.Basically, A fuse/breaker is supposed to break the connection between the user/applience and the high voltage.If both wires have high voltage to ground you fuse bothIf one wire has high voltage to ground and the other wire is ground then you only fuse the active wire.

author: Monkeh
author_rank: Super Contributor
timestamp: April 22, 2013, 09:30:09 am
content: Quote from: vk6zgo on April 22, 2013, 08:21:30 amIt's a no-no in distribution, but inside equipment, nobody cares. With some equipment (for example, equipment using IEC C7/8 connectors) polarity is not guaranteed anyway.

author: ptricks
author_rank: Frequent Contributor
timestamp: April 22, 2013, 12:55:00 pm
content: UL does not like to see neutral fusing and usually require very careful consideration when it is used because the engineer has to make sure that a blown fuse does not cause a code violation .example: You can fuse the neutral to a power supply, but if that power supply connects to chassis ground you cannot fuse the neutral because the PSU is always grounded and if the fuse should blow the PSU will use the ground as a neutral which isn't allowed.

author: Monkeh
author_rank: Super Contributor
timestamp: April 22, 2013, 01:27:02 pm
content: Quote from: ptricks on April 22, 2013, 12:55:00 pmAnd how would it do that without tripping every RCD it comes across by connecting neutral and ground?

author: Alana
author_rank: Frequent Contributor
timestamp: April 22, 2013, 02:31:31 pm
content: In my opinion internal fuse in device's PSU is not for protecting user from touching case that gone live but from protection against fire from internal short circuit so reversing live and neutral or fusing both power wires is not a problem as long as device is either double insulated or metal  parts that could be touched by user in normal operation of the device - properly grounded.In fact most extension cords with surge protection have fuses on both L and N and no regulatory commission makes any fuss about it.I think this sticker is more of informative to check BOTH fuses in case device does not work.

author: Monkeh
author_rank: Super Contributor
timestamp: April 22, 2013, 03:04:28 pm
content: Quote from: Alana on April 22, 2013, 02:31:31 pmMost Polish ones, I guess. Not once seen that.

author: somlioy
author_rank: Regular Contributor
timestamp: April 22, 2013, 05:51:28 pm
content: Quote from: ptricks on April 22, 2013, 12:55:00 pmWouldnt mechanically latched circuitbreakers fix that issue? That's the case in norway atleast. Not allowed to use singlepole circuitbreakers. So if one of the modules trip it takes the other phase with it. Same case with earth-leakage circuit-breakers. If it trips, it takes out both.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 23, 2013, 02:29:40 am
content: So what about nonpolarized wall warts? There's a 50% chance of plugging it in such that only the neutral is fused.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 23, 2013, 02:33:25 am
content: Wall warts and most other ungrounded appliances are designed double-insulated. No single insulation fault can connect the hot side to exposed metal. They can and should be switched and fused on both lines (if appropriate -- wall warts normally aren't switched at all).

author: ptricks
author_rank: Frequent Contributor
timestamp: April 28, 2013, 10:01:25 am
content: Quote from: Monkeh on April 22, 2013, 01:27:02 pmIt would trip RCD devices but RCD is rare in the US and only required in the past in wet areas like bath and kitchen areas. New homes have to have ARC fault or RCD .

author: ptricks
author_rank: Frequent Contributor
timestamp: April 28, 2013, 10:04:50 am
content: Quote from: somlioy on April 22, 2013, 05:51:28 pmIt depends on the device, how much of a load exists and the homes wiring. When it comes to wiring in homes there are a lot of variations since the code doesn't require upgrading older wiring to current standards except when the home is sold and sometimes even that doesn't require an upgrade.

author: ptricks
author_rank: Frequent Contributor
timestamp: April 28, 2013, 10:06:52 am
content: Quote from: NiHaoMike on April 23, 2013, 02:29:40 amNo need to fuse both connections. The role of a fuse in a wall wart is to protect from fire that might result from an internal failure. Fusing either side works in that situation.The other thing to consider is that fuses can fail prematurely and 2 fuses raises that chance. Manufacturers don't really like to have products returned for fuse failures, repair shops love it though:)

author: Monkeh
author_rank: Super Contributor
timestamp: April 28, 2013, 12:43:57 pm
content: Quote from: ptricks on April 28, 2013, 10:01:25 am..So, the device won't (for a number of reasons) connect neutral and ground. Which means it cannot use the ground as a return path.