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I've purchased a Schurter 4304.6090 power inlet for a project and am a bit confused by the fuse configuration. The inlet requires two fuses: one on the load side and another on the neutral side of the circuit. Is this just for redundancy, or am I missing something here?

I plan to use this for 120VAC (USA).

Here's a picture of the inlet, along with a crude wiring diagram:

Schurter 4304.6090 power inlet

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This device is rated for 250V. Assuming that you are in the USA/Canada, your 240V is actually two 120V lines, 180 degrees out of phase.

If your application is 120V hot/neutral/ground, you can simply wire in only one fuse holder, on the hot

Sorry, I misread the datasheet. I see now that the 6090 variant has the fuse holders already wired in.

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  • \$\begingroup\$ The plan is to use this with 120VAC. From what I can tell, I have to use both fuses. If I don't use a fuse on the neutral side, the neutral from the input will dead-end at the fuse holder (unless I hardwire it to bypass the fuse, or use some sort of fuse bypass). \$\endgroup\$ – RoccoC5 Dec 27 '13 at 23:05
  • \$\begingroup\$ Also protects against internal earth fault independent of supply polarity caused by reversible plug or incorrectly wired plug (L + N reversed). \$\endgroup\$ – Transistor Dec 9 '15 at 21:05
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Double pole fusing has pros and cons.

A fuse in the neutral can leave the appliance in an "unpowered but live" state after clearing a live to neutral fault which is not especially desirable. It's particually hazardous in situations where people working downstream of the protection point may regard working on wiring after performing single pole isolation as safe (UK electricians are certainly in the habbit of doing this, I suspect US ones are too).

On the other hand in a portable appliance placed on the worldwide market or even the european market you really don't know for sure which wire will be the neutral (think of a german or italian mains plug) or indeed if EITHER wire will be a neutral (think of a US 240V supply or a UK building site 110V supply).

I'm not sure about commercial equipment standards but I know that EN60601 (the medical electrical equipment standard) requires that live-neutral reversal be treated as "normal condition" which means if (part of) the purpose of the fusing is to protect against live-earth faults then double pole fusing would be needed.

Personally if it's going to be used on a portable enclosure to supply only equipment inside the enclosure I would just fit the two fuses and not worry about it. In other circumstances I might make a different judgement call and try to get it replaced with a model that only has single pole fusing.

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This switch is rated at 250V and conceivably it could be used on: -

  • European 230VAC single phase systems (phase and neutral) where only one fuse is required and there are likely available inserts that bypass one fuse and just leave the single fuse in the hot (live) wire. If this type of fuse carrier isn't available for this switched-inlet then this bullet answer doesn't apply.
  • North American 208V two (from 3) phase system (both lines hot) and will probably need 2 fuses.

I'm not ruling out any other good reasons!

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    \$\begingroup\$ In Europe, at least in the UK, 2 fuses DO hurt and are strongly discouraged : if the neutral fuse blows, the circuit fails live! This switch is likely to be illegal anywhere in Europe unless only ONE fuse is in circuit : the other slot then accommodates a (harmless, disconnected) spare fuse. I don't know the regulations surrounding American centre-tapped 220V systems. \$\endgroup\$ – Brian Drummond Dec 27 '13 at 22:08
  • \$\begingroup\$ @BrianDrummond - thanks Brian I'll amend my answer. But I believe on this type of switch-block you can purchase recepticles that bypass one of the fuses and just leave the remaining single fuse. \$\endgroup\$ – Andy aka Dec 27 '13 at 22:27
  • \$\begingroup\$ @BrianDrummond: good point about failing live if the neutral fuse blows - I hadn't considered that. \$\endgroup\$ – RoccoC5 Dec 27 '13 at 23:10
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    \$\begingroup\$ I've actually spent some time looking for specific legislation : there's plenty on house wiring (any fused neutrals are pre-1937 and tend to result in emergency callouts!) but I haven't found anything definite for appliances. But it was strongly drilled into me many years ago ... don't! \$\endgroup\$ – Brian Drummond Dec 27 '13 at 23:23
  • \$\begingroup\$ @BrianDrummond "it was strongly drilled into me many years ago ... don't!" that doesn't mean whoever drilled it into you was right in all contexts though. \$\endgroup\$ – Peter Green Dec 9 '15 at 18:29
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You don't see this every day!. The UL file http://www.schurter.ch/en/Documents-References/Approvals/%28id%29/6765+AND+FIELD+CertificationInstitute=UL (it's only an excerpt)

States

Conditions of Acceptability - snip

. These devices have been only considered for use as supplementary (not in lieu of branch circuit) overcurrent protection within appliances. The Part Nos. 1062, 1062X, 1064, 1064 B1, 1067, and 6766, which accommodate a European type fuse, have the capability of fusing in both grounded and ungrounded leads. Cat. No. 1064 B2 accommodates only one European type fuse. The acceptability of this construction should be determined in the end-use product.

The emphasis is mine and means that rules and regulation that apply to the final whole assembly will determine if and when you can use single or double fuses. Also the "not in lieu of branch circuit" means testing under fault conditions may require the fuses to substituted for nails/copper bar during UL testing ..... it's fun to watch the smoke (from both the device and the previously smug design engineers ears....)

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  • \$\begingroup\$ @DoxyLover answer is the reason why there are two fuses. \$\endgroup\$ – Spoon Dec 27 '13 at 22:42
  • \$\begingroup\$ I wonder how much it would cost to produce a double-pole fuse in such a way that failure of either leg would disconnect both? That would seem ideal from a safety standpoint. \$\endgroup\$ – supercat Dec 16 '14 at 1:20
  • \$\begingroup\$ It's common for breakers to be designed like that but I would imagine it would be very difficult to do it reliablly in a fuse. \$\endgroup\$ – Peter Green Dec 9 '15 at 18:15
  • \$\begingroup\$ @PeterGreen: I don't know how hard it would be; have two contacts which are spring loaded to open away from each other, joined by a piece of insulating material which is designed to fail at a certain temperature. If either contact gets too hot, the material will melt and both contacts will get pushed open by their respective springs. \$\endgroup\$ – supercat Dec 9 '15 at 18:56

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