Guys and Ladies, All this talk about Deltas, Wyes, panelboards ... I think we're missing the point here. For one thing, I assume that we're all talking about fuses inside a product. I've seen some responses dealing with circuit breakers... I leave that stuff up to the Electricians. I would assert that a product with a dual fused input (one fuse in the "hot" and one in the "nuetral") should meet safety requirements in any country as long as the rest of the product has been designed to meet all of the other safety requirements, such as insulation, grounding and ground conductor sizing. Remember, we're not interrupting the "nuetral" in the building wiring (which would be very, very bad). We're interrupting the nuetral within a product.
For instance, in some European 230VAC power grids, we can't be sure whether the Blue or Brown wire is hot or nuetral (when I say nuetral, I mean tied to Earth at the distribution panel). In this instance, I like dual fusing because I don't know which line will incur the overcurrent. A fuse in the assumed "hot" line may not catch the overcurrent. In these cases, I also like a two pole power switch on our products if we use a power switch. Now, lets consider a dual fused product with a grounded chassis used in the US where neutral is tied to ground. In this case, if a fault opens the fuse in the neutral line inside the product. Where does the return current go? It shouldn't be able to flow through the ground conductor, because basic insulation is required between nuetral and ground. The product should be disfunctional (telling the user that something is wrong) and it should still be safe with its grounded chassis. Isn't this product safe in this single fault condition? OK, let's assume basic insulation has been breached and the return current flows through the ground conductor (we now have a double fault). The product may function in an unsafe condition. However, isn't the chassis still grounded? Even with this double fault condition, the user is still afforded some protection. Depending on how much return current is flowing, the chassis may still not reach a hazardous voltage. Although it may give the user an unpleasant tingle, which should indicate to them that something is wrong. Now, lets consider a dual fused product with an insulated chassis (no ground connection and insulation meeting double or reinforced insulation requirements) in the US where neutral is tied to ground. In this case, if a fault opens the fuse in the neutral line, the product won't function. Where can the return current go? And, while the unit is "disfunctional", it still has its Class II insulation in place to protect the user. Isn't this product still safe in this single fault condition? The same two arguments above would apply in 230VAC systems with ungrounded nuetrals. So, if a product needs internal fusing to pass safety requirements, I believe that two pole fusing is a good way to ensure a safe product worldwide. However, in order for the dual fusing to be safe, the product needs to have proper safety construction, using either a grounded chassis (with properly rated ground conductor) or Class II insulation so that the product would remain safe in a single fault condition. Can anybody shoot a hole in this theory with a single fault condition? I'd love to know because it would help us all design safer products. This is my opinion and mine alone, not that of my employer. Chris Maxwell, Design Engineer GN Nettest Optical Division 6 Rhoads Drive, Building 4 Utica, NY 13502 PH: 315-797-4449 FAX: 315-797-8024 EMAIL: [email protected] > -----Original Message----- > From: Peter Tarver [SMTP:[email protected]] > Sent: Monday, October 09, 2000 3:30 PM > To: emc-pstc > Subject: RE: Overcurrent Protection: One or Both Sides? > > There seems to be some confusion regarding terminology and the location of > the overcurrent protective devices being discussed. > > In the US and Canada, if a circuit breaker is located in the panelboard, > John is generally correct that the neutral will not be controlled by the > circuit breaker. On the other hand, if the circuit breaker is located in > utilization equipment, there are no particular rules, regulations or > standards that I am aware of preventing one pole of a circuit breaker from > also controlling the neutral, except that, when the neutral is so > controlled, all phases must be disconnected by common a control mechanism. > > The common vernacular applied to supplementary overcurrent devices that > resemble branch circuit breakers is to also call them circuit breakers. > Even the manufacturers and distributors of such devices call them circuit > breakers. While incorrect in installation code terms, this is very likely > what most respondents are referring to. > > Regards, > > Peter L. Tarver, PE > [email protected] > > > -----Original Message----- > From: Wagner, John P (John) [ <mailto:[email protected]>] > Sent: Monday, October 09, 2000 3:28 PM > To: 'brian_kunde'; emc-pstc; 'Russell, Ray' > Subject: RE: Overcurrent Protection: One or Both Sides? > > > > I take some exception to the response below. > > Single phase connections between phases either on a delta or wye system do > > not have a neutral connection. > There are two grounded delta systems -- corner ground where one phase is > grounded, or center tapped ground on one of the phases (commonly called > high > leg). There are also variations of the grounding scheme to allow for > impedance grounded systems where appropriate. > > A standard two pole breaker os not designed, nor can it be used to > interrupt > neutral. For that, a special shunt trip braker is needed. > > John P. Wagner > AVAYA Communication > 11900 N. Pecos St, Room 2F58 > Denver CO 80234 > email: [email protected] > phone: 303 538-4241 > fax: 303 538-5211 > > > ---------- > > From: Russell, Ray[SMTP:[email protected]] > > Reply To: Russell, Ray > > Sent: Monday, October 09, 2000 5:15 AM > > To: 'brian_kunde'; emc-pstc > > Subject: RE: Overcurrent Protection: One or Both Sides? > > > > > > Hi Brian, > > > > I see most of the return postings have focused on the UK, which has a > > terminated neutral system. There are several installations especially in > > > the > > US, where the power could be derived from a delta, or unterminated > neutral > > system. I believe in this case, overcurrent protection is required on > both > > lines. In addition, the 2 pole circuit breaker method provides a > suitable > > 2 > > pole disconnect. > > > > Good Luck, > > > > Ray Russell > > [email protected] > > > > -----Original Message----- > > From: brian_kunde [ <mailto:[email protected]>] > > Sent: Friday, October 06, 2000 12:32 PM > > To: emc-pstc > > Subject: Overcurrent Protection: One or Both Sides? > > > > > > > > > > The issue of designing in Overcurrent Protection on one side or both > sides > > of > > the AC mains keeps coming up. We deal in Laboratory Equipment so the EN > > > 61010-1 > > is the standard we use. I'm not sure what EN60950 would say on the > > subject. > > > > The only area I have found that deals with this question is a NOTE in > > section > > 9.6 of EN61010-1 which says, "Overcurrent protection devices (e.g. > fuses) > > should > > preferably be fitted in all supply conductors." > > > > This seems "GRAY" to me and I get beat up on it all the time. I feel > that > > overcurrent protection should be on all current carrying conductors. > With > > a > > 230V~ product you never know where in the world the product will be > > shipped, > > if > > the AC Main has a grounded neutral, or if the receptacle is polarized. > > So, > > I > > feel you never know for sure which line or if both lines will be "HOT" > in > > reference to Earth ground. > > > > If my thinking is correct, shouldn't ALL 230V~ products have overcurrent > > > protection on both sides of the line? I would think so, but I see > > products > > everyday that only have ONE side of the line fused. My superiors feel > > that > > if > > others can get away with it, why can't we. Why add the extra cost of > > double > > pole breakers or double fuse holders if it is not necessary? > > > > Am I being too cautious or do I have a point? > > > > Thank you for your support and advice. > > > > Brian Kunde > > LECO Corp. > > > > > > > > ------------------------------------------- > > This message is from the IEEE EMC Society Product Safety > > Technical Committee emc-pstc discussion list. > > > > To cancel your subscription, send mail to: > > [email protected] > > with the single line: > > unsubscribe emc-pstc > > > > For help, send mail to the list administrators: > > Jim Bacher: [email protected] > > Michael Garretson: [email protected] > > > > For policy questions, send mail to: > > Richard Nute: [email protected] > > > > > > ------------------------------------------- > > This message is from the IEEE EMC Society Product Safety > > Technical Committee emc-pstc discussion list. > > > > To cancel your subscription, send mail to: > > [email protected] > > with the single line: > > unsubscribe emc-pstc > > > > For help, send mail to the list administrators: > > Jim Bacher: [email protected] > > Michael Garretson: [email protected] > > > > For policy questions, send mail to: > > Richard Nute: [email protected] > > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: [email protected] > Michael Garretson: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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