Re: What's a type "K" transformer?

There is no harmonic overloading on a 240/120 volt circuit. This does not occur in single phase panels.

transformers

Horse poo!..

Switch-mode power supplies, as installed in PC's printers, photocopiers and a vast array of other office equipment, generate quite a lot of harmonics (especially 3rd and 5th) back into the mains supply - more than enough to warrant full-size neutrals and de-rated transformers.

Cameron:-)

I've measured 100 amp/phase and 135 amps on the neural in a situation described by Cameron below (Computer Data Center).

SGT "Cameron Dorrough" a écrit dans le message de news: biultv$kq2$ snipped-for-privacy@merki.connect.com.au...

Unless the measurements were taken simutaniously with one meter capable of multiple measurements, or multiple similar meters (less accurite), it's difficult to pin this down with sequential measurements. The current levels are unlikely to remain static while you transfer a single instrument from one reading position to the other.

Louis

When we are talking harmonics, we usually are referring to the additive currents in 3 phase systems. Not single phase. It is common practice to use 2 wire circuits (one hot and one neutral) to eliminate harmonics in 3 phase panels.

Terribly sorry to disagree with you.. but taking a single-phase supply off a

Perhaps you could kindly explain to the group how high-frequency voltage components (ie harmonics) are suddenly not possible in a single phase system??

Thanks, Cameron:-)

On Tue, 2 Sep 2003 16:33:47 +1000, "Cameron Dorrough" Gave us:

If there is a sine wave involved, there are likely to be harmonics of that sine wave present.

On Mon, 1 Sep 2003 15:37:01 +1000, "Cameron Dorrough" Gave us:

Nice article..

The statement: "Therefore, these harmonics, when drawn from the single phase lines, can add together in phase to cause neutral currents that exceed the phase current. While the individual phase currents are less than the circuit breaker protection current, the neutral current caused by the triplen harmonics may EXCEED the current rating of the wire used." (emphasis added) is a trap for new-comers - especially offices and schools.

This a good reason why a licensed electrical inspector should check the design/installation of large single-phase installations *before* the power is turned on - or risk an electrical fire a few months/years down the track (and not necessarily at the switchboard either).

Cameron:-)

But an interesting point is that the neutral current can only exceed the phase current in multi-phase systems. In a residential scenario with only single phase supply, a lot of non-linear loads will definitely make a lot of harmonics. But the neutral current will only *equal* the 'hot' line current, not exceed it.

So as you say, reduced neutrals can be a hazard even in single phase installations. But full-size neutrals should never have a problem in single phase installation. In multi-phase installations, it is important to note that neutral current can *exceed* any individual phase current when there is high harmonic content.

Where there is both (i.e. a lot of single phase circuits run from a three-phase supply), the single phase circuits need full-size neutrals as the neutral current in the single phase branches will be exactly the same as the phase current. And where the single phase branches tie in to the three phase supply, *that* neutral may need to be oversized (IIRC, sqrt(2) larger) due to high harmonic content. Because where the single-phase neutrals are tied together and form the three-phase neutral is where the 'adding' takes place.

daestrom

Exactly.

I think the previous responders missed that very important point. I didn't feel the need to fuel the discussion any further.

Michael

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