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Basic MagneX™
Interrupter
Operation
March 1999 • New Issue • Cooper Industries, Inc.
FILE REF: 240-34
CP8802
TABLE OF CONTENTS
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PAGE
Electrical Schematic for a MagneX™ With/EO Installed in a Transformer
3
Sensor Element
4
Magnet Latch
5
Interrupting Chamber
6
Handle
7
Emergency Overload
8
Enable Emergency Overload
8
Disable Emergency Overload
8
History of the MagneX Handle
9
First Generation Handle
9
Second Generation Handle
9
Third Generation Handle Present Design
9
Conclusion
9
Float
10
MagneX Isolation Links
10
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ELECTRICAL SCHEMATIC FOR A MAGNEX™ WITH/EO INSTALLED IN A TRANSFORMER
HIGH VOLTAGE BUSHING
ISOLATION LINK OR ELSP FUSE
MAGNEX CONTACTS
MOVABLE CONTACT
EO RESISTOR
MAGNEX SENSOR
EO SWITCH
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SENSOR ELEMENT
The “HEART” of the MagneX™ is its unique sensing element. This sensor is sensitive to “primary” and “secondary”
faults and to excessive overloading of the transformer.
The sensor is able to sense these conditions through a very unique characteristic—its Curie Point. The Curie Point is
the temperature (approximately 200° C) at which the sensor changes from a ferromagnetic to a paramagnetic material,
or in layman’s terms, the point at which it loses its magnetic attraction and becomes nonmagnetic. This point is very
consistent and repeatable.
The sensor element is a nickel-iron alloy encapsulated in epoxy to hold it firmly in place. There are a number of different
sensor sizes to accommodate a wide range of primary voltages and kVA requirements. All sensors are made of the
same material; however, they vary in width and thickness to obtain the desired resistance.
FLOAT
INTERRUPTER
CHAMBER
BECAUSE THE MAGNETIC
LATCH IS SPRING LOADED, IT
WILL TRIP WHEN THE SENSOR
REACHES APPROXIMATELY
125°C.
SENSOR ELEMENT
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MAGNET LATCH
The magnet latch assembly is spring loaded and is typically magnetically latched to the sensor element. If the sensor
element reaches its Curie point, the magnet latch will lose its attraction for the non-magnet sensor element and will trip
the device because of the spring loading. When the device trips, the arc rod (lower movable contact) will open the circuit.
The magnet latch assembly will operate only upon electrical operation of the device. The magnet latch will stay magnetically latched to the sensor element during manual operation of the MagneX.
The magnet for the latch system uses a neodymium-iron boron type material, and is commonly called a “rare earth” magnet.
The characteristics of the magnet are very consistent and repeatable, the magnet has been extensively tested in our labs.
In addition, ferrite materials resist the demagnetizing effects of magnetic fields produced by transformer windings.
NORMAL POSITION
CONTACTS CLOSED
TRIPPED POSITION
CONTACTS OPEN
MAGNET LATCH
MAGNET LATCH
SENSOR
ELEMENT
MOVEABLE
CONTACTS
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INTERRUPTING CHAMBER
The interrupting chamber of the MagneX is where the circuit is opened and closed by movement of the arc rod inside
the chamber. The movable contact trip rod draws an arc from the high voltage contact ring in the oil inside the venting
interrupting chamber. The pressure generated drives liquid and the high pressure vapor through the elongated arc, so
that when current zero is reached, the dielectric strength builds at a very rapid rate, faster than the transient recovery
voltages can build up. This interruption mechanism is the same concept that has been used successfully in oil circuit
breakers. The following is a sequence of events that take place during interruptions:
1. The moveable contact trip rod draws an arc as it separates from the HV contact ring inside the interrupting
chamber.
2. The arc heats the oil in the chamber. With the current flow the oil is further heated, thereby gasifying the oil
and creating high pressure in the interrupting chamber.
3. The pressure generated will cause venting through the side exit. The HV contact plate will rise and release gas
from the top of the interrupting chamber.
4. When current zero is approached, the heating ceases, the arc cools and the interrupting chamber loses
pressure.
5. As current zero is reached, the arcing rod is moving. The gas will stop and rod tip will be surrounded by fresh
oil.
6. The fault has now been interrupted, and the MagneX Interrupter is able to withstand system voltage.
VENT
VENT
HV CONTACT
RING
VENT
INTERRUPTING
CHAMBER
MOVEABLE CONTACT
TRIP ROD (OPEN)
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HANDLE
The MagneX Interrupter can also be used as a handle operated loadbreak interrupting device. When the standard
handle is down, the contacts are closed. Rotating the handle upward causes the spring loaded contacts to open. Moving
the handle downward returns the device to the closed position.
In the closed position, this device will operate automatically if an overcurrent condition exists either due to a fault or an
overload. In this case, the circuit will be open with the handle pointed downward in the closed position.
The external handle does not move if the MagneX operates electrically. To reset, the handle must be pushed upward to
the open position and then back down to the closed position. Because the MagneX only requires five pounds of force to
operate manually, the handle is made of a flexible material to allow flexibility during hard operation. The handle is used
to energize, de-energize and reset a tripped MagneX.
Emergency overload is an option available with the MagneX. (Refer to page 8.)
OPENED
POSITION
N
PE
O
AL
RM
NO
ED
OS
CL
CLOSED
POSITION
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EMERGENCY OVERLOAD
The MagneX Interrupter can be readily switched to the emergency overload operation by rotating a small lever on the
main operating handle.
The emergency overload lever assembly restricts the travel of the main handle assembly which in turn closes a contact
switch on the rear of the MagneX body. This places a selected resistance in parallel to the sensing element so a portion
of circuit current will bypass the sensor element and thus desensitize it for overload conditions, producing a shift in the
long term trip characteristics.
ENABLE EMERGENCY OVERLOAD
Move handle to open position, turn emergency overload lever counterclockwise to “EO” position, move handle to closed
position.
DISABLE EMERGENCY OVERLOAD
NO
RM
AL
EO
CL
O
SE
D
O
PE
N
Move handle to open position, turn emergency overload lever clockwise to “Normal” position, move handle to “Closed”
position.
O
PE
N
DISABLE EMERGENCY OVERLOAD
NO
RM
AL
EO
CL
O
SE
D
EMERGENCY
OVERLOAD
RESISTOR
ENABLE EMERGENCY OVERLOAD
BACK VIEW WITH EMERGENCY
OVERLOAD
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HISTORY OF THE MAGNEX HANDLE
First Generation Handle
During 1985, over 100 MagneX interrupter units were manufactured for a field trial. These units were put on a utility
system and equipped with a metal index plate and metal handle. Two problems were encountered with the metal index
plate and metal handle.
• Tight tolerance control was necessary between the metal index plate, MagneX housing and tank hole sizing for
the Emergency Overload control.
• The metal handle projecting from the tank was susceptible to jarring during transporting or installation. Because
the metal handle was stronger than the switch shaft, the force transmitted to the shaft could break it. This would
require the transformer to be changed out and the MagneX replaced in a transformer shop. Oil leakage with its
subsequent clean-up problems could also result.
Second Generation Handle
• The index system for the emergency was incorporated into the switch block itself, removing the need
for close tolerance tank holes and mating index plates.
• The metal handle was changed to a thermoplastic handle designed to break before the switch shaft. The
strength of the hookstick eye was reduced so that it would break before any other part. The handle was also
designed to allow easy replacement in the field, if breakage did occur due to mishandling.
Field trial units shipped in early 1987 incorporated these revisions. There were still reports, however, that the thermoplastic handle could break if operated vigorously or from an awkward angle with a hookstick.
Third Generation Handle Present Design
The hookstick end of the handle was changed from a thermoplastic material to a flexible elastomer material with a slotted end. The flexible slotted end facilitated hookstick disengagement when operated vigorously or at an awkward angle.
The elastomer material remains flexible at temperatures as low as -40°C. Severe impact against the thermoplastic portion of the handle will cause it to break, thereby yielding before the shaft or switch block is damaged. The handle assembly is easily field replaceable.
The same UV stabilized materials have been used on the MagneX handles as those we have used for over 10 years in
other handles and caps.
Conclusion
The operating force of the MagneX interrupter is very low (less than 5 Ibs). The operating force required to actuate the
switch is less than 5% of the strength of thermoplastic portion of the handle. Experience with metal handles in the field
has shown the disadvantages detailed above.
The present handle is an engineered solution to the disadvantages already detailed. The hookstick end will flex and
open to accommodate excessive force during operation. If the body of the handle is overstressed, it will break before the
switch shaft or MagneX is damaged.
Therefore, the new handle is a true solution to several potential field problems, and it will protect the integrity of the
MagneX and the transformer.
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FLOAT
The MagneX Interrupter is supplied with a float assembly that mechanically prevents manual operation of the MagneX
handle under a low oil condition. The MagneX will still operate electrically with the float in the interlock position. This is a
safety feature to prevent a lineperson from mechanically operating a MagneX under a low oil condition.
FLOAT
MAGNEX ISOLATION LINKS
Either an isolation link or ELSP back-up current-limiting fuse must be placed in series with the MagneX. The isolation
link is used as an extra protective device when resetting a tripped MagneX. Isolation links are selected to melt when the
rating of the MagneX interrupter is exceeded. In the event of a transformer failure, a lineperson will not be able to close
the MagneX in on the fault.
Isolation links have no interrupting capacity. They are intended to operate only on high fault conditions and only to prevent re-energization. The use of a back-up current-limiting fuse can increase the interrupting rate to 50kA.
MAGNEX
ISOLATION
LINK (OR ALTERNATE BACK-UP
CURRENT-LIMITING FUSE)
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P.O. Box 1640
Waukesha, WI 53187
www.cooperpower.com
MagneX™ is a trademark of Cooper Industries, Inc.
MI
Printed on Recycled Paper
3/99