Center Tap, Neutral, Transformers

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joeyww12000

joeyww12000

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Chatsworth GA
Ok I have some more questions, Imagine that! This is for my boss who knows everything again. He told me you didnt have to bond a delta/wye 480 to 120/208 transformer because it created its own neutral, but you do have to bond a transformer that changes high voltage down to 240/120 because you would have a floating neutral. He's fooled alot of people over his 30 yrs of experience that he claims to have, luckily none of them are dead! Anyway I know he doesnt know what he's talking about, but Im not an expert scientific engineer on the way transformers work. What exactly does center tap mean, how does a transformer create a neutral from a scientific standpoint? Im here to learn and get better at my job not to be a fraud 30 yrs from now.
 
a "tap" is simply a spot where a wire is connected to the transformer winding. It can provide any voltage as designed. Some machinery has 240, 180, 90 volt etc taps as required.
A "center tap" is simply tapped from the center ofn the winding thus giving youn equal voltage to the other legs (240-120) (208-120)(480-277)
If you don't need that lower voltage then a transforemr with no tap can be used.
Bonding of the neutral on the transformer provides a stable voltage reference to ground and provides a return path to the transformer in the event of a ground fault.
I admire you for your desires to make yourself a better electrician and knowing when to question what you are told.
 
joeyww12000 said:
He told me you didnt have to bond a delta/wye 480 to 120/208 transformer because it created its own neutral, but you do have to bond a transformer that changes high voltage down to 240/120 because you would have a floating neutral.
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The same requirements apply to both, and for the same reasons. Either would be a floating system without bonding the neutral.

He probably also doesn't know that the new neutral needs to be bonded to the electrode system, using the same rules as a service.
 
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joeyww12000 said:
Ok I have some more questions, Imagine that! This is for my boss who knows everything again. He told me you didnt have to bond a delta/wye 480 to 120/208 transformer because it created its own neutral, but you do have to bond a transformer that changes high voltage down to 240/120 because you would have a floating neutral. He's fooled alot of people over his 30 yrs of experience that he claims to have, luckily none of them are dead! Anyway I know he doesnt know what he's talking about, but Im not an expert scientific engineer on the way transformers work. What exactly does center tap mean, how does a transformer create a neutral from a scientific standpoint? Im here to learn and get better at my job not to be a fraud 30 yrs from now.
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Throw his Kliens in the nearest canal, and hand him a broom!!
 
The neutral in a transformer is a point in the windings that has an equal potential with reference to the hot sides. By connecting this point to the earth the potential between this point and the surrounding enviorment will be zero.
 
If the neutral point of the secondary winding of a transformer is not earthed/grounded, then the output voltages will still be correct, for example 208 volts phase to phase and 120 volts phase to neutral will still be achieved.

BUT there will be no control over the voltages to ground, for example an insulation failure in the transformer could result in primary voltage of perhaps 4,160 volt being present on the output. This would clearly be dangerous.
With a correctly grounded secondary, the short to ground would instantly open the high voltage fuses.

Also if the secondary was ungrounded, a ground fault one phase would not be readily detected, and would result in 208 volts to ground on the other phases, which could overstress equipment designed for 120 volts to ground.
With a correctly grounded transformer, any phase to earth fault should promtly open the fuse or circuit breaker protecting it.

Therefore if the grounding connection to the neutral has been ommitted, or has dropped off, the installation would appear to work normally, but would be less safe than expected, especialy under fault conditions.
 
grantcool said:
Mr. Broadgage. you are obviously very knowledgeable about american electrical systems.
i'm curious - does england use our national electric code?
i thought everthing over there was 240 volt?
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The standard voltage over here is 230/400 volts officialy, but generally nearer 240/415 in practice.

UK codes (known here as the I.E.T. wiring regulations) are very different to those in the USA.
However some years ago I worked for a company that modified USA made electrical equipment for use in the UK.
I therefore had to learn a fair bit about the systems used in the USA, especially the less common ones.
UK systems are generally simpler, most UK electricians would never install a transformer for example (over than very small ones for 12 volt halogen lighting, they dont count as no real knowledge as required)

Large transformers are used to step down the primary voltage (typicaly 11,000 volts) to 240/415. These are installed by specialists engaged by the power company, not by regular electricians.
In some cases such transformers are privately owned, but installing or repairing would be considered a specialist job.

Virtually all services in the UK are either single phase 240 volts or three phase 240/415.
 
joeyww12000 said:
Ok I have some more questions, Imagine that! This is for my boss who knows everything again. He told me you didnt have to bond a delta/wye 480 to 120/208 transformer because it created its own neutral, but you do have to bond a transformer that changes high voltage down to 240/120 because you would have a floating neutral.
Click to expand...
You also have to bond the transformer enclosure/frame to the 480V EGC in case there is a short circuit from the primary to the enclosure/frame.
 
Also if the secondary was ungrounded, a ground fault one phase would not be readily detected, and would result in 208 volts to ground on the other phases, which could overstress equipment designed for 120 volts to ground.
Click to expand...

this looks like a good place for the open neutral picture showing the voltage differences on the phases and the resulting damage to equipment. anyone?
 
Transformer does not create the neutral

Transformer does not create the neutral

The transformer does not create the neutral - it gives the contractor the ability to ground the XO or center tap and the refrence to ground.

Since a neutral is a grounded current carring conductor, so even though the voltages are correct on the output of the transformer per the other posts - it can not be called a neutral till it is bonded.
 
broadgage said:
The standard voltage over here is 230/400 volts officialy, but generally nearer 240/415 in practice.
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The change to 230/400 was for "harmonisation" within the EU. With continental Europe being mostly 220/380 and UK being 240/415, 230/400 was a compromise. It practical terms, not a lot was changed so, as you say, 240/415 is usual. Meddlesome but not as bad as changing the 3-phase conductor colours. That was stupid and potentially lethal.
broadgage said:
UK codes (known here as the I.E.T. wiring regulations) are very different to those in the USA.
I think you possibly mean the IEE wiring regulations AKA BS7671?
And yes, different to the NEC.
Horses for courses, I suppose.
As you have noted, LV distribution in USA is quite different to that of UK.
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LarryFine said:
In other words, they have wye systems, not Delta.
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They are star of course.:)
Slightly more seriously...
The system has the merit that is simpler in that, for domestic wiring, you only ever deal with one single phase voltage, 230V line to neutral (derived from the 400V line to line). The idea of having your coffee-maker on a different voltage to your washer would be seen by many as a potential cause for confusion and error.

It's the same for commercial and light industrial. Where 3-phase low voltage* is used, it is the 400V. A high leg arrangement is unheard of here.

But we have a house in GA and the company I work for is owned by a US corporation, so I have had a bit of exposure to both UK and US systems.

*According to UK regulations, Low Voltage is defined as "not exceeding 1000Vac or 1500Vdc between conductors"
 
jrannis said:
Throw his Kliens in the nearest canal, and hand him a broom!!
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Throw his Kliens somewhere,
but hand him a Pen for his fingers, and wish him luck.

Take care of the boss, he does round up business and write checks.
 
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