Double Fusing - Page 1
Forum section: electronics / projects

author: Cicada
author_rank: Regular Contributor
timestamp: May 31, 2016, 06:48:46 am
content: HelloI have a question related to double fusing of IEC C14 power inlets. Look at the attached pdf for a diagram of the TN-S power system used at my facility. This is a simplified drawing but it captures the essence.My questions:1. Is it unsafe to have the fuse in the neutral phase of the equipment? Fuses F2,4,12,14 in the diagram.2. What could go wrong? Think wrong valued (different ratings) fuses inserted or one fuse blows before the other.3. Would the RCCB not cover all unsafe scenarios caused by double fusing?This article gives more info on RCCBs if you need to reed morehttp://www.cycloflow.com/2014/02/what-are-elcb-rccb-rcbo-and-rcds.html4. What does the wiring regulations says about a Neutral line fuse in TN-S systems? I don't have access to wiring regulations.5. Where (under what conditions) do they use fuses in the neutral line? Is it only in use cases where you cannot guarantee that your equipment will be used on a power outlet with phase reversal?See this link for Phase reversal and double fusing discussion. (http://incompliancemag.com/article/double-fusing-or-fusing-both-sides-of-the-line/)

author: Cicada
author_rank: Regular Contributor
timestamp: May 31, 2016, 07:28:08 am
content: Follow up on my original post.I have seen "internet article" that say neutral line fusing are prohibited by SOME wiring codesandThis bookhttp://bit.ly/1RHS9yceven tried to show why a neutral line fuse is dangerous. My guess is that the book was wrong because it did not consider having an RCCB in the circuit.

author: Fungus
author_rank: Super Contributor
timestamp: May 31, 2016, 07:31:04 am
content: Quote from: Cicada on May 31, 2016, 06:48:46 amIt's unsafe to have a fuse ONLY in the neutral phase.(technically: Unsafe if there's no RCCB)Quote from: Cicada on May 31, 2016, 06:48:46 amThere's no unsafe scenarios caused by double-fusing, every possible short circuit has a fuse in it (or two).Quote from: Cicada on May 31, 2016, 06:48:46 amIf the outlet plug can be reversed thensingle-fusing is dangerous, not double-fusing.

author: Fungus
author_rank: Super Contributor
timestamp: May 31, 2016, 07:33:25 am
content: Quote from: Cicada on May 31, 2016, 07:28:08 amIt should really be prohibited by ALL wiring codes. It costsnothingto put the fuse in the live wire instead of the neutral.Quote from: Cicada on May 31, 2016, 07:28:08 amWhat if the RCCB fails? A fuse in the neutral does nothing.

author: helius
author_rank: Super Contributor
timestamp: May 31, 2016, 07:38:10 am
content: Quote from: Cicada on May 31, 2016, 07:28:08 amThe book is wrong because it depicts touching a live wire as a normal activity. If equipment fails, you don't rely on the fuse having blown to make it safe to stick your hands inside it; you make it safe by unplugging it.If the context is where there is a chassis isolation fault, then it doesn't make any sense to suggest that the "neutral side" fuse would blow in this situation. Fault current that goes out the safety ground can only come in on the hot conductor, not on the neutral.

author: Fungus
author_rank: Super Contributor
timestamp: May 31, 2016, 07:51:31 am
content: Quote from: helius on May 31, 2016, 07:38:10 amNope. The book is correct because the world is full of idiots who might do just that.(How do you diagnose problems if it's unplugged?)

author: Someone
author_rank: Super Contributor
timestamp: May 31, 2016, 08:03:58 am
content: Quote from: Fungus on May 31, 2016, 07:51:31 amThose of us in overly protective counties require appliances to be safe in double fault condition. This requires proper earthing of all outlets of course.

author: ludzinc
author_rank: Unknown
timestamp: May 31, 2016, 08:19:11 am
content: Managing double fused equipment here in Oz in a real PITA.When you are working on a device that's may have a blown neutral fuse, you run the risk of shock as the active can still be conducting.At work we add warning labels and log items into a Dual Fused register.Neutral fusing is plain and simply illegal. Have had to have electrical work down by the government maintainers of the sites we work out of redone due to neutral fusing.

author: Fungus
author_rank: Super Contributor
timestamp: May 31, 2016, 08:39:18 am
content: Quote from: ludzinc on May 31, 2016, 08:19:11 amSee!(just kidding)Quote from: ludzinc on May 31, 2016, 08:19:11 amWhy would anybody eventhinkof doing that? ("It's OK, it'sfused!")(If that's "fused" then you're fired...)

author: Cicada
author_rank: Regular Contributor
timestamp: May 31, 2016, 09:05:08 am
content: Quote from: Fungus on May 31, 2016, 08:39:18 amWhy would anybody eventhinkof doing that? ("It's OK, it'sfused!")1. This is not a fixed installation. Like wiring to a motor in a factory or lighting. I want to fit these IEC power inlets to movable equipment. I have found phase reversal on some of out power outlets. Admittedly a big mistake (which I will try to rectify everywhere) but it does happen.2. I noticed in previous post that people talk about a fuse in the neutral line ONLY. That is certainly wrong and goes without saying. So a fuse in the live phase only with phase reversal gives you a fuse in the neutral wire. That is why double fusing is a viable option, I thinkThe question here is... are there any good reasons to not do a double fusing on non-fixed installed equipment.I don't care about redundancy. Only safety.I see it mentioned that some medical equipment standards requires double fusing.

author: H.O
author_rank: Frequent Contributor
timestamp: May 31, 2016, 09:13:39 am
content: QuoteThe fuses discussed are, to my understanding, the onesin the actual equipment, connected to the mains via a power outlet. Depending on where in the world you're located the power outlets and corresponding plugs may or may not be "rotatable", ie there's NO way to assure that a fuse on a single wire inside an appliance which have a power cord on it actually ends up on the phase and not on the neutral. It simply depends on which way the plug is inserted.I don't know rules, codes and regulations in, for example, the US but can you be SURE that all outlets are wired with the neutral in the same socket? I wouldn't bet on it.Fusing the neutral in thedistribution boardon the other hand - well that would be stupid.... but again, that's not the issue here as far I understand.

author: Cicada
author_rank: Regular Contributor
timestamp: May 31, 2016, 09:23:08 am
content: Quote from: H.O on May 31, 2016, 09:13:39 amYou are 100% correct. As my attached circuit diagram indicates the fuses are in the equipment and not in any of the circuitry, DB etc outside of the equipment.

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 31, 2016, 09:27:39 am
content: The reason for double fusing is due to the American system for providing 220V from a 110V supply.The 220Vac supply is generated by an anti-phase winding of 110Vac. The 220Vac equipment is connected between the normal and anti-phase wires so sees a potential of 220Vac. Each input however is only 110Vac relative to Neutral/earth. This connection style means that the equipment input terminal that is normally connected to the Neutral terminal in a 220Vac system is now connected to a 110Vac feed and hence requires fusing. Sounds confusing, I know.

author: Cicada
author_rank: Regular Contributor
timestamp: May 31, 2016, 10:56:26 am
content: Quote from: Richard Head on May 31, 2016, 09:27:39 amThat is interesting. Never came across any mention of such a scenario. Then again I live in a 220VAC country. It makes sense.Do you think that might then be the main reason why you get double fused power inlets in general? Such as the one in the attached document. To mount on equipment that will receive power from such a source. Or maybe both this issue and phase reversal in normal 220VAC supplies are both equally important enough to provide such power inlets. My guess is both are equally important.

author: TassiloH
author_rank: Regular Contributor
timestamp: May 31, 2016, 12:14:24 pm
content: Regarding double fusing for IEC connectors:Thin about the German Schuko plug, which is not polarised, so you don't know which pin is phase and which is neutral.For a fault between the phase and neutral wiring in your device, a single fuse in the device will protect your wiring, regardless if t ends up to be in the neutral or phase line.Now think about a fault to earth between any phase or neutral wire in your device and ground/earth. There are plenty of outlets without RCD in the world, so you can't assume an RCD is there which would trip. So you can a) make every wire and PCB trace in you device at least 0.75mm2 so that you know for sure the circuit breaker of the outlet, which should not exceed 16A (or 20A in the US, but polarised plugs there) will protect you from fire. Or b) put a fuse in both supply lines.And regarding the dangers: Why should it be dangerous to use double fusing in a device? A blown fuse does not mean a device is safe to open and touch the wiring anyway, there could always be unfused parts of the wiring and components protected by a second fuse that is not obvious...Its a different story of course if someone puts a separate breaker in the neutral line in a distribution box, that is a huge no-go: Neutral must never be disconnected without the corresponding phases in a 2- or 3-phase system, otherwise you end up with a floating neutral that takes on a voltage determined by the loads on the different phases, so overvoltage damage to connected devices is likely; in a 1-phase system this won't happen, but then turning off a breaker on a power panel and the corresponding circuit remaining hot is quite obscure and dangerous.

author: Monkeh
author_rank: Super Contributor
timestamp: May 31, 2016, 12:25:22 pm
content: Quote from: Richard Head on May 31, 2016, 09:27:39 amFixed that for you.

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 31, 2016, 12:45:11 pm
content: Let me re-phrase that statement.The reason for double fusing is due to the way American system produces 220Vac. Hopefully less confusing!

author: helius
author_rank: Super Contributor
timestamp: May 31, 2016, 04:03:27 pm
content: Quote from: Richard Head on May 31, 2016, 12:45:11 pmThe same considerations apply to single-phase circuits derived from three-phase distribution, of course. And that type is very common in European installations.

author: prasimix
author_rank: Unknown
timestamp: May 31, 2016, 04:23:36 pm
content: Quote from: Cicada on May 31, 2016, 10:56:26 amI think that double fusing also makes sense when AC input is secured with MOVs like in picture 12, or not?

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 01, 2016, 06:15:55 am
content: No, double fusing is not required in that instance since there is still only one mains feed. The fuse is there to prevent a fire. It's not intended to protect the equipment from damage. In any case, my experience indicates that MOV's provide better protection if they are connected directly to chassis and not via a gas tube.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 01, 2016, 09:18:43 am
content: Fuses are to protect against consequential damages when the devicehas already failed.You can use suicide resistors as fuse, which is what they do in cheap smps supplies.

author: wraper
author_rank: Unknown
timestamp: June 01, 2016, 09:46:09 am
content: Quote from: Jeroen3 on June 01, 2016, 09:18:43 amThey are proper fusible resistors unless it is some unsafe Chinese crap.

author: Cicada
author_rank: Regular Contributor
timestamp: June 01, 2016, 11:26:02 am
content: Quote from: Cicada on May 31, 2016, 10:56:26 amHi Richard.I googled for information on how they go about generating 220Vac from 110Vac in the US using this anti-phase winding. I could not really find a good source of information. Maybe I am searching using the wrong terms. Could you point me to a good source of information.Thanks

author: richard.cs
author_rank: Super Contributor
timestamp: June 01, 2016, 11:59:04 am
content: Quote from: Cicada on June 01, 2016, 11:26:02 amThe term you need to search for is "split phase", try starting here:https://en.wikipedia.org/wiki/Split-phase_electric_power

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 01, 2016, 01:05:39 pm
content: Yes, split phase is the correct term. Wiki has all the relevant info.

author: Cicada
author_rank: Regular Contributor
timestamp: June 01, 2016, 01:56:13 pm
content: Quote from: Richard Head on May 31, 2016, 09:27:39 amQuote from: richard.cs on June 01, 2016, 11:59:04 amBrilliant thank you guys. I think the picture is becoming clear now. This video explains 'split phase' 'anti phase'. Both descriptive names seem correct to me.The double fused power inlets that I am usinghttp://bit.ly/1P4ZurIhave no L and N stamped on the plastic of the connector like the single fused inlets that I also have. It is obvious that the single fused units should have the L and N stamped on it so that you do not accidentally do a phase reversal when wiring them. You will then have a single fuse in neutral only. That is if you are careless and do not adhere to the iec 60320 standard. See attachediec_60320_plug_and_socket_definitions.pngandiec_60310-1_c13_c14_pin_definitions.pngThe double fused units do not have L and N stamped on the body of the housing but it does have L and N (in the correct orientation according to iec 60320) on the mains filter sticker.So in summary. It seems that the double fused IEC power inlets are mainly used:1. in such 'split phase' systems where you want a fuse in both phases because both are actually at some voltage above safety earth.2. in non-'split phase' systems where you (as equipment manufacturer) cannot guarantee that there will be no phase reversal from the supply side where the product is used. So live will always be fused. And I think in non-'split phase' systems you should be safe with an additional fuse in the neutral (although redundant) and an upstream RCCB.Feel free to complete the picture. Thank you very much for everyone's contribution.