Line/load side of equipment

[big john]

I am not aware of any formal definition of how to know which side is primary and which side is secondary, in all applications....

The primary is always the side connected to the source of power. That is the NEC definition as well.

That said, that is not necessarily a fixed thing, especially in generation schemes where power may flow through a tranny in either direction. That's why we often tefer to the windings as "high" and "low" to avoid confusion.

 

[ATSman]

Line/ Load

Line/ Load

I have seen many cases where a MCCB is reverse fed (mounted upside down.)
In the case where a breaker has an under-voltage device then it cannot be reverse
fed, otherwise the breaker would not close since the UV device would not be energized (with
breaker open.)

 

[GoldDigger]

The other criterion that can be used to identify the primary is the source of the magnetizing current when the transformer is energized. For some transformer configurations the magnetizing surge is very asymmetric so it makes a difference.
For transformers that will be used for bidirectional power transfer the preferred construction gives more symmetric magnetization.
When a stepdown transformer is used for PV backfeed the two potential definitions give conflicting results.

 

[Carultch]

When a stepdown transformer is used for PV backfeed the two potential definitions give conflicting results.

My point exactly. Is primary the side that remains energized after the other disconnect is in the open position? Or is primary the side from which power flows during normal operation?

From what I understand, is that the NEC resolves the ambiguous case is by requiring you to satisfy rules pertaining to either side considered as primary. But it doesn't say whether 240.21(C) has to apply to both opposite sides as if they were transformer secondary conductors.

 

[Carultch]

That's why we often refer to the windings as "high" and "low" to avoid confusion.

Because voltage is based upon the winding ratio, which is a function of internal geometry. And it will give you a definite high and low voltage side of the same device no matter which way it is used in a bidirectional application.

 

[wwhitney]

The other criterion that can be used to identify the primary is the source of the magnetizing current when the transformer is energized. For some transformer configurations the magnetizing surge is very asymmetric so it makes a difference.

So when a transformer is used bidirectionally, is there something akin to a startup surge each time the power flow changes direction?

Thanks, Wayne

 

[GoldDigger]

So when a transformer is used bidirectionally, is there something akin to a startup surge each time the power flow changes direction?

Thanks, Wayne

No. The phase of the applied voltage is not changing nor is it going away momentarily, so the magnetizing current is continuous.
The surge is primarily a single cycle phenomenon.

 

[Tom176]

Reverse Feeding

Reverse Feeding

It is very dangerous to reverse feed equipment. Not as an equipment issue, but as an issue for other fellow electrical workers. There was an incident a work involving an arc flash, because of a reverse fed breaker. It was properly labeled to indicate the Line and Load, but sometimes people just don't read signage. Did you ever see someone pushing on a door that was clearly marked pull? It goes for everything. Please make every attempt possible not to reverse fed equipment. Otherwise, you may be setting someone else up to get killed.

 

[Carultch]

It is very dangerous to reverse feed equipment. Not as an equipment issue, but as an issue for other fellow electrical workers. There was an incident a work involving an arc flash, because of a reverse fed breaker. It was properly labeled to indicate the Line and Load, but sometimes people just don't read signage. Did you ever see someone pushing on a door that was clearly marked pull? It goes for everything. Please make every attempt possible not to reverse fed equipment. Otherwise, you may be setting someone else up to get killed.

Most transformers and blade disconnects are suitable for backfeed, because there is no reason they can't carry power in either direction. With breakers, you need to verify if it is suitable for backfeed.

A lot of breakers are suitable for backfeed, but then again, there are also a lot of them that are not suitable for backfeed. Unfortunately, to verify that one is, it is like trying to prove a negative to research it. Prove that it isn't marked "LINE" and "LOAD", rather than a datasheet clearly indicating whichever case it may be.

 

[jumper]

Most transformers and blade disconnects are suitable for backfeed, because there is no reason they can't carry power in either direction. With breakers, you need to verify if it is suitable for backfeed.

You have to verify that the tranny is suitable for backfeed also. The tranny has to be marked for it.

450.11
(B) Source Marking. A transformer shall be permitted to
be supplied at the marked secondary voltage, provided that
the installation is in accordance with the manufacturer’s
instructions.

Never backfeed a blade/knife switch/disconnect.

404.6
(C) Connection of Switches. Single-throw knife switches
and switches with butt contacts shall be connected such that
their blades are de-energized when the switch is in the open
position.

 

[Carultch]

You have to verify that the tranny is suitable for backfeed also. The tranny has to be marked for it.

450.11
(B) Source Marking. A transformer shall be permitted to
be supplied at the marked secondary voltage, provided that
the installation is in accordance with the manufacturer’s
instructions.

Never backfeed a blade/knife switch/disconnect.

404.6
(C) Connection of Switches. Single-throw knife switches
and switches with butt contacts shall be connected such that
their blades are de-energized when the switch is in the open
position.

The power can still originate on the load side of a knife blade switch, provided that the power source is de-energized by the action of the switch moving to the open position. In fact, that is how you are supposed to connect open blade switches in PV applications. Line side toward the grid, because it is permanently voltage-energized. Load side to the inverter sources, because elimination of grid voltage causes them to shut off. Grid-tied inverter sources look for a stable grid voltage first, and then inject their power.

Breakers that are marked line/load by contrast, cannot be used in this manner with a power source on the load side. Because they are not listed for safe operation when backfed.

 

[GoldDigger]

Breakers that are marked line/load by contrast, cannot be used in this manner with a power source on the load side. Because they are not listed for safe operation when backfed.

Even then, there is a practical difference between a situation where, for example, the load terminals of a GFCI breaker remain energized when the breaker is open and the situation where there will be no voltage present once the breaker opens but the direction of power flow is different while it is closed and operating normally.
But the code does not explicitly recognize that the two situations are different.
Ultimately it is up to the manufacturer to correctly label the device based on what problems can arise with their particular design under both conditions.

 

[iwire]

Even then, there is a practical difference between a situation where, for example, the load terminals of a GFCI breaker remain energized when the breaker is open and the situation where there will be no voltage present once the breaker opens but the direction of power flow is different while it is closed and operating normally.

:huh::huh:

 

[GoldDigger]

:huh::huh:

Three cases to consider;
1. The breaker will for some reason not function properly when the direction of power flow is opposite normal. At the moment I can't think of anything but a network protection relay that would behave that way.
2. For some reason the interrupting action of the contacts depends on which side is energized. Hard to see why this would happen except for DC.
3. The most common, namely that the trip mechanism might fail if power remains applied to it after the contacts open. Some shunt trip wiring may behave this way.
Of these three, only the first one is a practical concern when the situation involves a grid interactive inverter. But any of the three will lead a manufacturer to add line/load labelling, preventing it from being used for PV backfeed as well as power backfeed to an MLO panel.

 

[iwire]

Three cases to consider;
1. The breaker will for some reason not function properly when the direction of power flow is opposite normal. At the moment I can't think of anything but a network protection relay that would behave that way.
2. For some reason the interrupting action of the contacts depends on which side is energized. Hard to see why this would happen except for DC.
3. The most common, namely that the trip mechanism might fail if power remains applied to it after the contacts open. Some shunt trip wiring may behave this way.
Of these three, only the first one is a practical concern when the situation involves a grid interactive inverter. But any of the three will lead a manufacturer to add line/load labelling, preventing it from being used for PV backfeed as well as power backfeed to an MLO panel.

OK, that made things less clear.

First you talked about a GFCI that had live load side terminals when open. When does that happen in a properly wired system.

So I was confused, now you respond with something which seems unrelated to a GFCI or this thread in general. :huh:

 

[GoldDigger]

The question has been posed before whether a GFCI breaker (not receptacle) can be reverse fed. Not sure why anyone would do that, but I imagine all GFCI breakers have identified load terminals. Reverse wiring a GFCI receptacle would definitely be a wiring error.