Transcript What can a Non Load Break Switch do?

What can a Non Load Break
Switch do?
Interesting question
A switch is used to isolate the circuit
Can a switch close into an energized
circuit, a circuit with load on it, a circuit
with a fault on it?
Can a switch open an energized circuit, a
circuit with load on it, a circuit with a fault
on it?
Answer
1. Can a switch close into an energized circuit—Yes
2. Can a switch close into a circuit with load on it –Yes
3. Can a switch close into a circuit with a fault on it –
Yes
4. Can a switch open an energized circuit without any
load – Depends
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
load break device on it.
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
the line is de energized by the breaker/fuse.
The question that does not have a
definite yes or no is question 4.
Because over the years we have bought a
number of switches without load break devices
on them.
What is load? Well it could be a long line with
charging current, it could be an unloaded
transformer, it could be the breaking of parallel.
So how much is too much for an energized
circuit to be considered as unloaded or loaded
therefore requiring a load break.
Lugoswr.mpg
Two things that affect a circuit
breakers ability to interrupt a
circuit
1. The amount of current it has to
interrupt. The more current the harder it
is to open.
2. How fast the voltage builds across
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
Want it to arc until current zero
Contact part
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
will not interrupt the circuit?
Because during the arcing time from contact
part to current zero there is a conductive
media made that has to be removed as soon
as current zero otherwise there will be no
dielectric established to withstand the voltage
between contacts after the current goes to
zero. The more current the more conductive
media made and the more that must be
removed.
We are playing a game, we are trying to
remove as much conductive media as we can
as soon as current zero and pull the contacts
as far apart as we can before the voltage can
built. This voltage is called the transient
recovery voltage (TRV)
For a resistive circuit the voltage
and current are in phase
Voltage across switch contacts
Voltage 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.
Capacitor Voltage
Current
System voltage
Voltage
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
can establish my dielectric verses how
fast the voltage builds
Besides pulling the contacts apart very fast I can help
myself by bottling up the arc until current zero then
letting the arc energy move the dielectric media very fast
into where the arc was.
Breakers are good at this, but an air switch has very
limited capabilities because the arc is in air and not
contained as in a breaker, plus the air is a variable, could
be wet, hot, low pressure day etc. A function of how the
lineman switches and how fast.
So you have to be careful on what you can interrupt with
a switch without a designed interrupter.
So what does all this mean?
A Breaker is designed to be able to handle
the arc until current zero and then be able
to handle the TRV across the contacts
after the arc goes out.
An Air switch with a load break device is
able to handle the arc also up to its rating
However it is air switches that have no
way of handling the arc that we have to be
aware of what we are switching.
So what kind of switches do we
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 Interrupter
Whip interrupter
Load Break Bottle
Circuit Switcher is capable of interrupting fault current
So what are we trying to
sometimes do with Air switches that
are not load 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
non load break air switch
There is not a controlled media to make the
break.
Manufacturer’s do not test their switches for
ratings as there are too many variables
Manufacturer’s do not recommend interrupting
any current with hook stick switches or fuses
and very low levels with group operated air
switches with arcing horns
We have some guidelines of what we can do
with disconnects that were reached from
operating experience.