# Centre tapped transformer

Could someone point me in the direction of information regarding centre tapped transformers. I need more details to try and understand why they are
used in the uk on mains voltage for controlling reduced voltage equipment. I understand primary windings and that the purpose of the secondary winding is to reduce or increase the voltage however for what reason is a transformer centre tapped. As I understand it if the input voltage is 230v and the secondary winding outputs 110v and a centre tap is placed in the middle of the secondary winding this will produce 55v. Question is why not ensure that the windings on the secondary are suffice to only produce 55v and not 110v. I think I am missing a principle here.
Any info greatly appreciated.
Regards
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Peter G wrote:

Relative to what? The way this is setup you get effectively 55-0-55 giving 110V across the tool motor but only 55v to earth which reduces the chances of a bad shock.

Cable size! at 55V you need double the current and that means cables have to be larger. Also each volt dropped is a greater percentage of the total so that also makes the cables larger....
Regards, Dan.
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wrote:
| Could someone point me in the direction of information regarding centre | tapped transformers. I need more details to try and understand why they are | used in the uk on mains voltage for controlling reduced voltage equipment. | I understand primary windings and that the purpose of the secondary winding | is to reduce or increase the voltage however for what reason is a | transformer centre tapped. As I understand it if the input voltage is 230v | and the secondary winding outputs 110v and a centre tap is placed in the | middle of the secondary winding this will produce 55v. Question is why not | ensure that the windings on the secondary are suffice to only produce 55v | and not 110v. I think I am missing a principle here.
The reasons depend on how things are connected.
For starters, center tapping, and grounding the center tap, at any voltage level cuts the electrocution risk voltage in half. Your utility power in UK is 230 volts to ground, while over here in the American colonies, we have 240 volts center tapped so the ground potential is only 120 volts. If you touch a live wire while grounded you get whacked with 230 volts in UK, but only 120 volts in US.
The 110/55 system you are looking at probably first has to be addressed in terms of why it exists in the first place.
If the loads are 110 volts, then in theory a 220/110 system could have been used. But if the usage involves more exposed wiring or is being used in more risky places like swimming pools, then getting the voltage as low as feasible is an advantage.
If the loads are 55 volts, then in theory a 55/27.5 system could have been used and gotten an even lower ground potential. So presumably electrocution risk isn't an issue for 55 volt loads on a 110/55 system.
Sensitive electrical equipment like studio audio amplifiers works better, with less noise coming in from the power wiring, if the configuration is center tapped with the equipment powered from both opposing phases (to cancel out common mode noise). A company in the US makes equipment that provides a 120/60 volt system to run 120 volt audio equipment with to keep noise levels reduced. The side effect of a slightly safer 60 volt ground potential is not the design goal. Such a system could be done in the UK with a 230/115 volt setup, center grounded, and run the equipment that is designed for 230 volts there. It is necessary for the audio equipment involved to not assume either power conductor is grounded for this to work. We have a slight advantage over you in that we can use your 230 volt audio equipment on our 240/120 volt system directly and get the same effect :-)
If you can provide more information on exactly how the 110/55 volt system is actually hooked up to its loads, we might be able to explain more.
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are
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not
There are a couple of different things to consider.
Perhaps the *most* important, is how many different voltage levels do you need? If you need just one voltage level, to supply a dedicated load, then you don't need center tap at all. If you need dual voltage levels, then you can either use two secondary windings, or use one winding with a tap somewhere between them depending on the two voltages you need. If needed voltage happens to be exactly double of the other, then a tap in the exact center is appropriate. If you use a center tap, then you can evenly distribute the low-voltage loads between the center tap and each 'end tap'. Thus, you can maximize the utilization of the transformer windings.
Another consideration, is if it is a high-voltage to low-voltage step down transformer, you probably should ground some part of the secondary circuit for safety. If a primary-secondary fault should occur, you could have high voltages present on the secondary where it could shock someone or start a fire. In the US, in commercial applications, many forms of grounding have been devised (corner-delta, center wye, even specially connected grounding-transformers). But the common residential connection in the US is to ground the center tap. So you get 120V from one 'end' and the center and 120V from the other 'end' and the center. No conductor is more than 120V above ground. If a primary - secondary fault develops, then current from the primary will flow into secondary winding and to ground. The high current will trip the primary-side protection and at no time is the secondary side subject to primary line voltage.
Why dual voltage? Well as I understand it, in places like the UK all the 'mains' are just one voltage. It certainly can be done. Is 240V *really* that much more dangerous than 120V? Not really, either one can kill you if you don't show it the proper respect. Higher voltage would mean less current and I^2R losses. Of course, too high and you start to have problems with insulation.
But the so-called 'Edison' connection harkens back to the days of the first Edison generating stations that generated DC. Such generators were actually outfitted with extra brushes that allowed a 'neutral' to be connected in. So a 240V DC generator would have a 'positive', 'neutral' and 'negative' lead. Between 'positive' and 'negative' was 240V, between 'positive' and 'neutral' was +120V and between 'negative' and 'neutral' was -120V. Interestingly, this same electrical setup was also used on board some ships of the period.
Once there was a large number of heaters and light-bulbs for 120V/240V, there was no going back. It became a de-facto standard in the US and lives on today (with minor changes to the exact voltages over the years). Kind of like why railroad rails are four feet, eight and one-half inches apart.
daestrom
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if
Yes, it "really" is much more dangerous.
Just a first order guess would have it twice as dangerous but it is more than that.
For example, 24 VAC is considered harmless. What this means is that the FIRST 24 volts is a "freeby." Thus, your 120 volt shock is less than 100 volts over the "freeby" voltage. But your 240 volt shock is more than 215 volts over the "freeby" voltage.
But telephone experience has demonstrated that 48 volts is actually a "freeby" level. If so, 240 volts is (240-48)/(120-48)2/72 times as dangerous.
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On Tue, 11 May 2004 20:25:55 -0400, "John Gilmer"

In an electrical system, safety is not the ONLY consideration. It may, very well be the major consideration, however.
To the person who suggested a 55 - 0 - 55 volt scheme. Yes this would indeed be safer, as far as accidentical electric shock is concerned. However, the voltage drop with such a low level voltage would be unacceptable without increased expense. (An additional reason is that many electronic devices require a ground at zero volts, the same as the zero neutral (center-tapped) potential.
Increased voltage drop = wasted power and far shorter runs of low voltage distribution lines at practical voltages.
Most of Europe and a large part of the rest of the world has decided that a hot wire and 240 or 250 volts and a neutral is the way to go. A 240 v. circuit can be run a much longer effective length than a 120 volt circuit, and the advantages of existing standardization, lower voltage drop, and higher efficiency outweighs the increased danger to human life.
One the other hand, the single phase 240/120 system as developed in the US offers both safety (120 V. common lighting and applicance circuits) and the flexibility of a choice of two voltages with minimal voltage drop and a common neutral. (Remember in a typical US residential installation, from distribution transformer to electric meter, the neutral current will be much less than the current in the hot legs, zero if the two legs are ideally balanced, but lower in most any case.) This aids in keeping the voltage drop mostly confined to a 240 volt circuit. Higher voltage = more efficiency plus less wasted power.
The best of both worlds.
Beachcomber
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|> Why dual voltage? Well as I understand it, in places like the UK all the |> 'mains' are just one voltage. It certainly can be done. Is 240V *really* |> that much more dangerous than 120V? Not really, either one can kill you | if |> you don't show it the proper respect. | | Yes, it "really" is much more dangerous.
I disagree about the "much" part. The reason is that the common shock mode is line to earth ground, more often due to a fault putting a line voltage on a chassis (and a grounding wire not present). Very rare is the shock by touching twp line wires at the same time across a large distance of the body (and through the heart).
Since line to earth ground is 120 volts, due to the center tap, then the most common shock hazard is mitigated by that lower voltage.
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You are making far too many assumptions. A very significant cause of deaths by electricity is in fires caused by conductors/connections/contacts overheating, for which 120V is 4 times worse than 240V.
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Andrew Gabriel

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the
*really*
215
But your 'freeby' type of analysis works on the upper end as well. Touch 120V while standing in puddle of water and you die. Touch 240V while standing in puddle of water and you die. So both are just as *deadly* when standing in a puddle of water. Similarly, use a class 00 glove to work on either one and you won't feel anything.
Like I said before, "either one can kill you if you don't show it the proper respect." Neither one is likely to jump an air gap and zap you from even a 1 inch distances. Both can create a large enough flash/arc to throw molten metal/parts at you.
From a practical standpoint, they *both* present the same hazards.

This is silly. Yes, the current would be 192/72 higher the exact same conditions. But that is like arguing a truck hitting you at 70 mph (that has (70^2)/(55^2) the kinetic energy of a 55 mph one), is *more* dangerous than the 55 mph. Wrong, you're just as dead regardless of which truck hits you. The only difference is how wide the splatter.
It's thinking like that that explains why OSHA has seen a disturbing increase in electrical *fatalities* of experienced electrical workers that are working on *low* voltage systems. The *experienced* workers figure the lower voltage isn't as dangerous and end up dead.
daestrom
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when
Un huh.
What about touching the circuit will standing on damp concrete on a hot day (sweat) and touching the wire. At 120 you might "feel a tingle." At 240, if your are lucky, you are knocked on your ass.
Voltages are grounding schemes are selected so that something going wrong doesn't automatically that the next guy down the trail ends up dead.

proper
a
molten
Well, electricians who work with more than 150 volts to ground have to be a LOT more careful: not only for thier own protection but becuse of the spectacular effects of the higher voltage faults. This is both a practical and a safety issue. That's why control circuits are usually 120 or 24: mistakes just don't do as much damage.

Nonsense.
hits
Yes, but a car hitting you at 25 will not kill you (most of the time) and that's why urban speed limits are kept that low on many city streets.

the
Accidents happen. But, at the expense of an occasional mistake, electricians can work quickly and efficiently around 120-0-120 systems. Much equipment is rated for less than 150 volts above ground. That's why 120-0-120 is a GOOD system.

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Touch
on
day
240,
even
a
practical
Does 240V jump out and 'grab ya'? Does it require a different insulation class? Is 120V *not* lethal? I guess if you want to try and measure pain-thresholds, there's a difference. But when working around it, they both require the same safety precautions, tools, PPE.

as
(that
dangerous
We lost a cop a few years ago while he was directing traffic. Inadvertantly stepped over the yellow line into the wrong lane of traffic. Car was going 28 mph (as determined post-accident), cop was wearing reflective vest, broad daylight. Knocked him 30 feet and killed him instantly. Just less splatter than 55.
Much like 120V, can kill you just as dead, just fewer burns and smaller 'holes'.

that
But the point is, *experienced* electrical workers are having *more* accidents on low voltage systems than five years ago. Partly because when working on low voltage, they think it is 'safer'. They are thinking, apparently just like you, that lower voltage means 'safer', and more of them are ending up injured or *dead*.

why
Very few things have insulation rated for 120V that cannot withstand 240V or even 277V. Even computer power supplies and other electronics are more and more rated for multi-national use up to 277V. The same insulation classes used in motors for 120V can withstand 240V with no problem (but the motor has to be re-connected or re-wound). Even old 'varnish' insulated equipment can withstand 480V. Biggest problems would be switching devices and their problem isn't voltage to ground, but voltage/current to be interrupted.
If it's such a good system, why doesn't Europe, Australia and many other parts of the world use it? Because it *doesn't* make that much difference.
daestrom
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Touch
on
day
240,
In my company, if you touched the circuit without having a lockout/markup and doing a live-dead-live test, you'd be fired on the spot. Or if you *have* to work it hot, you'd be wearing gloves so you wouldn't 'feel' a thing.
Or are you one of those 'old-timers' that checks the voltage levels with the back of your hand? We had a guy like that. Didn't want to follow the rules cause "he'd been working around electricity for 20 years and hadn't been hurt yet". We fired him.
daestrom
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| What about touching the circuit will standing on damp concrete on a hot day | (sweat) and touching the wire. At 120 you might "feel a tingle." At 240, | if your are lucky, you are knocked on your ass.
Why would it be any different? Both have a 120 volt potential between the wire and ground. The center tapped transformer sees to that. You get 120 volts and 240 volts from that very same wire, depending on where the other connection is made.
Or are you referring to the system in Europe and much of Asia where the ground potential is from 220 to 240 volts?
Or are you referring to the 277 volt circuits in industrial locations in the US (347 in Canada)?
| Well, electricians who work with more than 150 volts to ground have to be a | LOT more careful: not only for thier own protection but becuse of the | spectacular effects of the higher voltage faults. This is both a practical | and a safety issue. That's why control circuits are usually 120 or 24: | mistakes just don't do as much damage.
At a given kVA capacity, a transformer with 2 times the voltage will have only 1/2 the current, and thus 4 times the impedance. That will result in only half the available fault current if the transformer is the bulk of the impedance of the circuit.
However, one usually sees higher voltage used where substantially more power is needed, and so you will see more spectacle if there is a fault. But that's not because of the voltage specifically, but because of the power available.
I believe control circuits are usually 120 or 24 volts because it is more practical to build them for a market with a variety of voltages. If you had control devices for 480 as well as 240 and 120, then things just get more expensive with too many different versions of things.
| Yes, but a car hitting you at 25 will not kill you (most of the time) and | that's why urban speed limits are kept that low on many city streets. | |> |> It's thinking like that that explains why OSHA has seen a disturbing |> increase in electrical *fatalities* of experienced electrical workers that |> are working on *low* voltage systems. The *experienced* workers figure | the |> lower voltage isn't as dangerous and end up dead. | | Accidents happen. But, at the expense of an occasional mistake, | electricians can work quickly and efficiently around 120-0-120 systems. | Much equipment is rated for less than 150 volts above ground. That's why | 120-0-120 is a GOOD system.
But the 120-0-120 system presents the SAME hazard whether one touches the black wire OR the red wire. And that's the same whether those wires are the ones going to a 240 volt air conditioner, or to a 120 light fixture.
But also, for the reason 120-0-120 is a good system, 60-0-60 is a better system, if your 120 volt equipment doesn't assume one of the hot wires is a grounded neutral wire. With 60-0-60, touch either the black wire or the red wire and you only get slapped with 60 volts, versus being smacked with 120 volts if you touch the black wire or the red wire of 120-0-120, or being whacked with 277 volts if you touch the brown wire or the orange wire or the yellow wire of 480Y/277.
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wrote:
| It's thinking like that that explains why OSHA has seen a disturbing | increase in electrical *fatalities* of experienced electrical workers that | are working on *low* voltage systems. The *experienced* workers figure the | lower voltage isn't as dangerous and end up dead.
Perhaps those that don't make "contact live wires" mistakes earlier in their career end up figuring they are safe and take more risks "because I've never have any problem with it before and now I'm very experienced with this". It's mistakes that teach the most. But with electricity, such lessons are also very expensive.
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This link should give you the required info
http://www.hse.gov.uk/lau/lacs/18-3.htm
Hope it helps

are
winding
230v
not
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| This link should give you the required info | | http://www.hse.gov.uk/lau/lacs/18-3.htm | | Hope it helps
Interesting.
Maybe there should be an effort to adopt a world-wide common voltage and wiring configuration for usage in this category now while we still can and get it to be at least accepted for usage in the safety codes in various countries. I propose to cut that voltage in half yet again.
So the voltage should be in the range 50 to 63.5 based on a multiple of two ratio of the local voltage standard, 100 -> 50 in Japan, 220 -> 55 in Britain (note, half of what is mentioned now), 230 -> 57.5 in Europe, 120 -> 60 in US, 127 -> 63.5 in Mexico, etc. The secondary would be center tapped to make half that voltage to ground on single phase (so that range will be 25 to 31.75), and wye/star configured on three phase for 1/1.732 of the voltage to ground (28.9 to 36.7).
So Europe would see it as 57.5/28.75 single phase, 57.5/33.2 three phase. And USA would see it as 60.0/30.0 single phase, 60.0/34.6 three phase.
Equipment (like hand operated tools) designed for this system should be required to function on both 50 or 60 Hz, and over the full range of 48 to 64 volts, obtained only from the 2 or 3 hot wires only, never from the neutral (which will not be available). A grounding conductor will be required.
The plug/receptacle should be 2 or 3 pins enclosed in a coaxial frame that prevents access to the pins while being inserted into the receptacle. The coaxial frame will be a solid contact grounding conductor. A 2 pin plug shall fit in a 3 pin receptacle at any of 3 angles, and obtain 2 of the 3 hot legs. A 3 pin plug shall not fit in a 2 pin receptacle (where the 3rd hole would be is thus blocked).
I envision a plug that looks somewhat like a GR-874, with 6 outer blade serving as ground, and 2 or 3 inner pins for single or three phase. The ground blades will be longer than the inner pins.
Ground fault protection is required to open all hot lines if any leakage exceeding 2 milliamps total is detected (whether to the grounding conductor or to earth ground). Any equipment trying to use the ground wire as a neutral conductor shall trigger the ground fault interruption.
| |> Could someone point me in the direction of information regarding centre |> tapped transformers. I need more details to try and understand why they | are |> used in the uk on mains voltage for controlling reduced voltage equipment. |> I understand primary windings and that the purpose of the secondary | winding |> is to reduce or increase the voltage however for what reason is a |> transformer centre tapped. As I understand it if the input voltage is | 230v |> and the secondary winding outputs 110v and a centre tap is placed in the |> middle of the secondary winding this will produce 55v. Question is why | not |> ensure that the windings on the secondary are suffice to only produce 55v |> and not 110v. I think I am missing a principle here. |> |> Any info greatly appreciated. |> |> Regards |> |> | |
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The problem would be to " adopt a world-wide common voltage and wiring configuration" Europe is slowly geting to a sort of European voltage standard after years of trying! How long a world standard will take is anyones guess!

they
equipment.
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| The problem would be to " adopt a world-wide common voltage and wiring | configuration" | Europe is slowly geting to a sort of European voltage standard after years | of trying! | How long a world standard will take is anyones guess!
I didn't mean for the regular mains power. I meant for the low voltage power used for the specific circumstances the posting described. Since it would be an all new kind of thing, the opportunity to do it in a world standard way exists until someone adopts a different way.
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