Guest Tony Norton Posted June 1, 2005 Share Posted June 1, 2005 Hello all Can somebody explain to me exactly what is "reverse polarity"? My distant memories of ONC(Elec) tell me that AC is a sine wave with an amplitude of 220V/240V. If derived from a "Star" 3 phase supply it's "neutral" is the centre of the star and it's "line" (commonly, but incorrectly in my view, called "live") is taken from one of star points. (The 440V 3 phase supply for heavy machinery being drawn from all three star points only.) If I am right, and I'm probably not as I'm thinking back nearly 50 years, the "line" oscillates at 50 cycles/sec from 0V to 240V whilst the neutral does the same, but opposite phase to the "line". If this is so why does "polarity" matter? If you plug in a 2 pin plug for a shaver, audio equipment and sundry other gear, it doesn't matter which way round the plug goes, so how can "polarity" matter? I could understand it if we were dealing with nasty, dangerous high voltage DC. When we were sailing we used to plug our shore line into the outlets in the French marinas without questioning the polarity, and never blew up any of our gear, although we do know that some people used to carry "cross-over" cables. Were we just lucky? Is there a 'clued up' electrician out there who could help me with a detailed explanation please? All the Best Tony N Link to comment Share on other sites More sharing options...
Guest David Tester Posted June 1, 2005 Share Posted June 1, 2005 The polarity of the live & neutral is very important for the following reason, the live line in the 230v AC supply is at 230v with respect to earth, the neutral line is as close to zero volts as it can be with respect to earth. If single pole switching is used on appliances (this is only one of the conductors being disconnected by the switch)it is the live line that is switched, if the polarity is reversed before the switch then the neutral is switched, this means that if a fault occurs the appliance could be at 230v potential with respect to earth when switched off and could have fatal results. Best Regards, Dave Tester Link to comment Share on other sites More sharing options...
Guest Gordon Posted June 10, 2005 Share Posted June 10, 2005 Can I bring in the observed reason as too anybody who has ever removed the back off a television or radio or electronic component that the unit does not actually use 240v as a whole but is stepped down via a transformer to smaller, say 12vdc,6vdc,3vdc voltages then wired of to the components that power/work the telly/radio etc. So to bring in the positive power(brown) up the wire(blue)of the electrical component is like feeding our bodies via our backsides(sorry). Some electrical components resent this and won`t work again and die. So we say how do the European`s solve this problem,i`ve never looked into it.Possible wire blue/brown reversal inside the electrical appliance ???? I`ve only ever found the pos/neg reversed in France so far.with earth thats another story !! Gordon. Link to comment Share on other sites More sharing options...
Guest Tony Norton Posted June 10, 2005 Share Posted June 10, 2005 Hello Gordon What you say is deifinitely true of DC voltages, but AC does not quite work the same. If it did we would never be able to purchase electrical items with 2 pin plugs, e.g. 100V/220V shavers, chargers for battery powered shavers/toothbrushes etc.etc. When 240V AC is stepped down, via a transformer for example, it then has to rectified via a diode(half wave rectification), or a diode bridge (full wave rectification) and then smoothed using a combination of capacitors and an inductor. The rectification circuitry doesn't care what phase the transformer is outputting, so reversing the 240V AC input of a bit of kit that has transformer/rectifier voltage conversion wouldn't blow up the gear. I think David's safety comment is valid, but thanks for taking the trouble to respond. Tony N Link to comment Share on other sites More sharing options...
Guest PeteC Posted June 14, 2005 Share Posted June 14, 2005 Actually revers polarity has nothing at all to do with Ac v Dc But is the result of the connections Star v Delts at the generator windings With star each coil is connected at the Neutral end - with the lives coming from the oposite end The 'Star Point' is connected to earth - as used in the UK With delta there is no star point the coils are connected in series with the ends all being conected as a greek Delta and it is possible for the leads at the supply point to be rongly connected The result could affect SOME electronic gear - but may also result in some earthed equipment having a live case It is only a few years since I retired from the trade & have tought elecrical supply for some 35 years Link to comment Share on other sites More sharing options...
Guest Tony Norton Posted June 15, 2005 Share Posted June 15, 2005 Thanks Pete You helped me dredge up some distant memories from my ONC(Elec) days by mentioning Star v Delta. I think I now see the light. Thanks again Tony N Link to comment Share on other sites More sharing options...
Guest PeteC Posted June 15, 2005 Share Posted June 15, 2005 Dont mention it Tony If I hadnt been teaching the subject I wouldnt have remembered We carry a short length of cable - long enough to reach the groung without any weight on the cables - with a plug on one end & socket on the other (with the wires reversed) and just insert that into the line if the detector gives a reversed reading Actually we had to remove the earth wires from some electronic equipment in a researh rig to cut out voltages built up in the earth loops So perhaps the continental system is better in some ways The only place we have found reversed polarity is in Spain & before you can check it the man has usually scarpered with the key to the box Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.