What can a Non Load Break What can a Non Load Break Switch do?Switch do?

Interesting questionInteresting question

A switch is used to isolate the circuitA switch is used to isolate the circuit

Can a switch close into an energized Can a switch close into an energized circuit, a circuit with load on it, a circuit circuit, a circuit with load on it, a circuit with a fault on it?with a fault on it?

Can a switch open an energized circuit, a Can a switch open an energized circuit, a circuit with load on it, a circuit with a fault circuit with load on it, a circuit with a fault on it?on it?

AnswerAnswer

1. Can a switch close into an energized circuit—Yes1. Can a switch close into an energized circuit—Yes2. Can a switch close into a circuit with load on it –Yes2. Can a switch close into a circuit with load on it –Yes3. Can a switch close into a circuit with a fault on it – 3. Can a switch close into a circuit with a fault on it – YesYes4. Can a switch open an energized circuit without any 4. Can a switch open an energized circuit without any load – Dependsload – Depends5. Can a switch open a circuit with load on it –Yes if it 5. Can a switch open a circuit with load on it –Yes if it has a load break device on it – No, if it does not have a has a load break device on it – No, if it does not have a load break device on it.load break device on it.6. Can a switch open a circuit with a fault on it – No this 6. Can a switch open a circuit with a fault on it – No this is the job of a breaker or fuse. The switch can open after is the job of a breaker or fuse. The switch can open after the line is de energized by the breaker/fuse.the line is de energized by the breaker/fuse.

The question that does not have a The question that does not have a definite yes or no is question 4.definite yes or no is question 4.

Because over the years we have bought a Because over the years we have bought a number of switches without load break devices number of switches without load break devices on them.on them.

What is load? Well it could be a long line with What is load? Well it could be a long line with charging current, it could be an unloaded charging current, it could be an unloaded transformer, it could be the breaking of parallel. transformer, it could be the breaking of parallel. So how much is too much for an energized So how much is too much for an energized circuit to be considered as unloaded or loaded circuit to be considered as unloaded or loaded therefore requiring a load break.therefore requiring a load break.

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Two things that affect a circuit Two things that affect a circuit breakers ability to interrupt a breakers ability to interrupt a

circuitcircuit1. The amount of current it has to 1. The amount of current it has to interrupt. The more current the harder it interrupt. The more current the harder it is to open.is to open.

2. How fast the voltage builds across 2. How fast the voltage builds across the switch contacts.the switch contacts.

We always want to interrupt on a current zero. Why?

If we force the current to zero before a natural current zero then the slope of that line di/dt could be infinite so voltage across inductors v=Ldi/dt will be to high! Current Chop

Contact part

Want it to arc until current zero

So if we want it to always interrupt at current zero why do we So if we want it to always interrupt at current zero why do we say a breaker has an interrupting rating which when exceeded it say a breaker has an interrupting rating which when exceeded it will not interrupt the circuit?will not interrupt the circuit?

Because during the arcing time from contact Because during the arcing time from contact part to current zero there is a conductive media part to current zero there is a conductive media made that has to be removed as soon as made that has to be removed as soon as current zero otherwise there will be no dielectric current zero otherwise there will be no dielectric established to withstand the voltage between established to withstand the voltage between contacts after the current goes to zero. The contacts after the current goes to zero. The more current the more conductive media made more current the more conductive media made and the more that must be removed.and the more that must be removed.We are playing a game, we are trying to We are playing a game, we are trying to remove as much conductive media as we can remove as much conductive media as we can as soon as current zero and pull the contacts as soon as current zero and pull the contacts as far apart as we can before the voltage can as far apart as we can before the voltage can built. This voltage is called the transient built. This voltage is called the transient recovery voltage (TRV) recovery voltage (TRV)

For a resistive circuit the voltage and current are in phase

Voltage across switch contactsVoltage across switch contacts

This is an easy circuit to interrupt

For a inductive circuit the voltage and current are 90 deg out of phase.

Voltage across switch contacts

This is a lot harder to interrupt

For a capacitive circuit the voltage and current are 90 deg out of phase.

Voltage

Current

Voltage

System voltage

Capacitor Voltage

This is easy to interrupt, but can easily restrike and can cause bad things to happen

Remember it is a game as to how fast I Remember it is a game as to how fast I can establish my dielectric verses how can establish my dielectric verses how

fast the voltage buildsfast the voltage buildsBesides pulling the contacts apart very fast I can help Besides pulling the contacts apart very fast I can help myself by bottling up the arc until current zero then myself by bottling up the arc until current zero then letting the arc energy move the dielectric media very fast letting the arc energy move the dielectric media very fast into where the arc was.into where the arc was.Breakers are good at this, but an air switch has very Breakers are good at this, but an air switch has very limited capabilities because the arc is in air and not limited capabilities because the arc is in air and not contained as in a breaker, plus the air is a variable, could contained as in a breaker, plus the air is a variable, could be wet, hot, low pressure day etc. A function of how the be wet, hot, low pressure day etc. A function of how the lineman switches and how fast.lineman switches and how fast.So you have to be careful on what you can interrupt with So you have to be careful on what you can interrupt with a switch without a designed interrupter. a switch without a designed interrupter.

So what does all this mean?So what does all this mean?

A Breaker is designed to be able to handle A Breaker is designed to be able to handle the arc until current zero and then be able the arc until current zero and then be able to handle the TRV across the contacts to handle the TRV across the contacts after the arc goes out.after the arc goes out.An Air switch with a load break device is An Air switch with a load break device is able to handle the arc also up to its ratingable to handle the arc also up to its ratingHowever it is air switches that have no However it is air switches that have no way of handling the arc that we have to be way of handling the arc that we have to be aware of what we are switching.aware of what we are switching.

So what kind of switches do we So what kind of switches do we have?have?

No Arcing Horns on either the hook stick switch or fuse, this switch/fuse is not rated to interrupt any current

Arcing Horns

This switch has arcing horns so if there is any arc on opening or closing it is on the arcing horn instead of tearing up the contacts.

High Velocity InterrupterHigh Velocity Interrupter

Whip interrupter

Load Break Bottle

Circuit Switcher is capable of interrupting fault current

So what are we trying to sometimes So what are we trying to sometimes do with Air switches that are not do with Air switches that are not

load breakload break

Drop lines

Break Parallel- could be a bus tie or loop switching

Could be this

Or this

But in either case the voltage on each side of the switch is the same

Magnetizing Current

So what does this all mean for a So what does this all mean for a non load break air switchnon load break air switch

There is not a controlled media to make the There is not a controlled media to make the break.break.Manufacturer’s do not test their switches for Manufacturer’s do not test their switches for ratings as there are too many variablesratings as there are too many variablesManufacturer’s do not recommend interrupting Manufacturer’s do not recommend interrupting any current with hook stick switches or fuses any current with hook stick switches or fuses and very low levels with group operated air and very low levels with group operated air switches with arcing hornsswitches with arcing hornsWe have some guidelines of what we can do We have some guidelines of what we can do with disconnects that were reached from with disconnects that were reached from operating experience. operating experience.