Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Reexamination Certificate
2001-04-16
2003-10-21
Toatley, Jr., Gregory J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
C361S104000, C361S117000
Reexamination Certificate
active
06636409
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to apparatus for protecting a facility and its loads from surges and other disturbances in supply voltage and, more particularly, to a surge protection device such as, for example, an alternating current (AC) surge protective device (SPD) for such loads. The invention also relates to devices and methods for manufacturing a thermal fuse spring.
2. Background Information
Various devices and arrangements are known for protecting loads from surges in supply voltage. Generally, two approaches are used: series and parallel protection. In series protection, a high impedance is used in series with the load during a surge to block or limit surge current. In parallel protection, the surge current is diverted with a low impedance shunt. The present invention provides parallel protection.
It is known to use voltage clamping devices and crowbar devices to provide parallel surge protection. The voltage clamping devices clamp the voltage across the load to a specified level. Common types of clamping devices include metal oxide varistors (MOVs), zener diodes and silicon avalanche diodes. If used alone to provide parallel protection, an MOV, for example, must have a clamping voltage that is above the nominal supply voltage in order that its maximum continuous operating voltage (MCOV) is above the nominal supply voltage. Known crowbar devices include gas discharge tubes and thyristors. These devices normally have a high impedance and switch to a low impedance when a surge in the supply voltage exceeds the breakdown voltage of the gas in the case of the gas discharge tube, or is high enough to activate a trigger circuit to turn the thyristor on.
FIG. 1
shows a prior proposal for a surge protection device (SPD) circuit in which thermal disconnection of a failing MOV is used. The SPD
2
includes a conventional overcurrent fuse
4
, three MOVs
6
,
8
,
10
, and two thermal cut-off(TCO) devices
12
,
14
. The SPD
2
is employed in a conventional AC power circuit
16
including a power input
18
having phase
20
, neutral
22
, and ground
24
connections, and a power output
26
having phase
28
, neutral
30
, and ground
32
connections. A circuit breaker or fuse is typically employed upstream from the SPD
2
, in order to protect against a fatal failure of the SPD. For example, a circuit breaker or switch
34
is electrically connected in series between the phase input connection
20
and the phase output connection
28
.
There are several disadvantages of this prior proposal. First, the reaction time is relatively long. The metal cases of the TCOs
12
,
14
are warmed by heat generated by the MOVs
6
,
8
,
10
, with heat transfer from MOV to TCO being provided by radiation through the air. Because air and the MOV coatings are thermal isolators, it takes time for the TCOs to disconnect the MOVs during an abnormal condition of the SPD
2
. Second, the surge current rating of the TCOs
12
,
14
is also a limiting factor. One TCO is typically suitable for only one MOV. Third, the major disadvantage is cost, since one TCO costs several times more than the corresponding MOV, which is to be protected. In order to improve the cost ratio, it is known to employ a combination of a plural MOVs connected in parallel in combination with a single TCO. However, in that case, the TCO does not have a surge current capability equivalent to that of the parallel connected MOVs.
FIG. 2
shows a SPD
36
including a plurality of MOVs
38
, which are electrically interconnected in parallel. Each of the MOVs
38
is electrically connected in series with a corresponding one of the fuses
40
in order to provide individual overcurrent protection. Each of the fuses
40
is intended to open circuit when the corresponding MOV
38
fails and electrical current flowing through that fuse exceeds the current rating thereof. However, that current rating might not be suitable to prevent fire on the corresponding MOV
38
in the event that the surge rating of the corresponding fuse
40
exceeds the surge current rating of that MOV. For example, in the event of a catastrophic failure of one or more of the MOVs
38
, the SPD
36
, and perhaps the protected load (not shown), might be subject to substantial damage from fire and/or explosion. Degradation of MOVs might occur due to various reasons, which might lead to such a catastrophic failure of the SPD. Although this rarely happens, the end user must be protected.
A relatively high surge-rated SPD usually has plural MOVs connected in parallel in order to share surge currents. A failure of MOVs is extremely rare, but it is catastrophic when it occurs. The reasons for such failures are most often due to abnormal overvoltage (e.g., up to about 200% of nominal voltage) or surges.
There are several known proposals for fusing of MOVs in SPDs. For example, it is known to employ a fuse trace (FT) copper conductor on a printed circuit board (PCB) in series with a single MOV, or overcurrent protection per phase (e.g., a standard RK5 fuse, fuse resistor, block fuse, TCO, thermal fuse, surge fuse). However, such fuse traces are functional only up to certain levels of surge current. For example, known fuse traces cannot handle relatively higher surges (e.g., higher than about 6.5 kA) and still remain suitable for disconnecting a failing MOV at a fault condition (e.g., a fault current of less than about 10 A). This presents two opposite requirements for fuse trace design. First, in order to handle relatively higher surge currents, a fuse trace must have a cross-sectional area which is as large as possible. Second, in order to disconnect a failing MOV in a relatively short time, a fuse trace must have as small of a cross-sectional area as possible. Overcurrent protection, alone, is insufficient to meet these two requirements.
Recently, MOV surge ratings are increasing to relatively higher levels. For example, several years ago, 20 mm MOVs were rated up to about 6.5 kA (with a 8×20 &mgr;s standard waveform). Today, 20 mm MOVs are rated up to about 18 kA surge current.
U.S. Pat. No. 4,862,134 discloses an electrical fuse including a fusible conductor welded or bonded onto separated contact surfaces.
U.S. Pat. No. 6,067,216 discloses a circuit arrangement for protecting an electrical component from an undesirable electrical potential. The circuit arrangement includes a protective element, such as a diode, varistor or thyristor, in parallel with the electrical component. When a protective action occurs in response to an inadmissible voltage surge, the protective element, such as a surface mounted diode, experiences breakdown and becomes shorted which causes the fuse to open shortly thereafter. Also, the solder mounting the diode becomes liquefied as a result of heating of the diode. A divided pair of soldering pads is employed for one of the connections to the diode, which results in a more rapid rise of temperature than on an undivided soldering pad for the other connection to the diode.
U.S. Pat. Nos. 5,600,295 and 5,896,080 disclose thermal fuses for circuit substrates and printed circuit boards.
Generally, fusing in SPDs can be internal or external (e.g. inside the SPD enclosure, or circuit breakers and fuses outside the SPD enclosure).
There is a need for improvement of the fusing function in surge protection devices.
SUMMARY OF THE INVENTION
A surge protection device includes plural voltage clamping means, such as MOVs, accompanied by a thermal fuse spring and a fuse trace. Preferably, the three components are coordinated in terms of MOV (pre-fault) leakage current, MOV fault current, and MOV surge current rating for safe disconnection of the MOVs under certain failure conditions, such as overvoltage or surge.
As one aspect of the invention, a surge protection device for a load supplied with at least one voltage from a power source comprises: at least two terminals adapted to receive the voltage; and a plurality of surge protection circuits, each of the surge protection circuits comprising: a thermal
Bandura Mieczyslaw
Funke James
Ghanem Mahoud
Ha Chi Thuong
Kladar Dalibor
Eaton Corporation
Moran Martin J.
Rodriguez Isabel
Toatley , Jr. Gregory J.
LandOfFree
Surge protection device including a thermal fuse spring, a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Surge protection device including a thermal fuse spring, a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Surge protection device including a thermal fuse spring, a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3170769