Failsafe surge protector having reduced part count

Details Failsafe surge protector having reduced part count Failsafe surge protector having reduced part count

2002-03-28

2003-08-12

Sircus, Brian (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

High voltage dissipation

C361S118000

Reexamination Certificate

active

06606232

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to surge protectors of the type commonly used on telecommunications lines to divert voltage surges to ground, and more particularly, to a failsafe surge protector having a reduced part count.
2. Description of the Related Art
Surge protectors are well known for protecting personnel and telecommunications equipment by diverting voltage surges, also known as overvoltages, on a telecommunications line to ground. Such surge protectors utilize various types of protection elements to divert unacceptable levels of voltage to ground, including an air gap, a gas tube, and a metal oxide varistor (MOV) or other solid state device (e.g., thyristor or bi-directional voltage switch). A surge protector utilizing a single protection element is sometimes reliable and provides sufficient protection against the level of voltage surges encountered. Oftentimes, however, a surge protector utilizing more than one type of protection element is required to provide redundancy or to improve the performance of the surge protector. For example, an MOV may be used in conjunction with a gas tube as a back-up protection device to provide continued protection to personnel and equipment in the event that the gas tube fails (e.g., the gas tube vents). When used as a back-up protection device, the MOV has a clamping voltage at a preselected current (e.g., 1 mA) that is greater than the DC breakdown voltage of the gas tube. An MOV may also be used in conjunction with a gas tube as a hybrid protection device to reduce the reaction time of the surge protector or to reduce the impulse breakdown voltage of the gas tube without permitting the MOV to burn out. When used as a hybrid protection device, the MOV has a clamping voltage at a preselected current (e.g., 1 mA) that is less than the DC breakdown voltage of the gas tube.
It is also known to provide surge protectors with a failsafe mode of operation. A surge protector provided with a failsafe mode of operation continues to protect personnel and equipment in the event that the primary protection element overheats, or both the primary protection element and the secondary protection element overheat. In a particular failsafe surge protector, the protector assembly is provided with a fusible element having a predetermined melt temperature. If the temperature of the fusible element reaches the predetermined melt temperature, the fusible element melts and provides an electrical short-circuit path between the telecommunications line and ground. A commonly utilized fusible element is a solder pellet made of a fluxed metal alloy that has a predictable melt temperature and transitions rapidly between the solid state and the liquid state. The melt temperature of the fusible element is selected based on the temperature at which the protection element overheats (or is otherwise rendered inoperable), the thermal conductivity of the protection element, and the location of the fusible element in the surge protector relative to the protection element.
A known failsafe surge protector including a gas tube, an MOV, and a fusible solder pellet for protecting the tip and ring conductors of a telecommunications line is shown in
FIGS. 1A and 1B
. The surge protector, indicated generally at
110
, includes a non-conductive housing
112
defining an internal cavity. Tip and ring line terminals
114
extend outwardly from the cavity through openings formed in the top surface of the housing
112
. Each line terminal
114
is threaded to receive fasteners
113
and spacers
115
for securing and separating one or more tip and ring wire pairs that are electrically connected to the line terminals
114
. The surge protector
110
further includes a pair of voluted springs
116
, a pair of gas tube assemblies
118
(FIG.
1
A), a common ground cap
119
, and a common ground terminal
120
. The voluted springs
116
, the gas tube assemblies
118
and the ground cap
119
are disposed within the cavity defined by the housing
112
and are secured within the cavity by the legs
121
of the ground terminal
120
, which engage the underside of the ground cap
119
through openings formed at the bottom of the housing
112
. The underside of the ground cap
119
and the legs
121
of the ground terminal are typically encased with a potting compound (not shown) to seal the internal cavity along the bottom of the housing
112
against environmental contaminants, such as dirt, dust and moisture.
Each voluted spring
116
is electrically connected to one of the tip and ring line terminals
114
. The gas tube assemblies
118
include identical sets of tip terminal protection elements and ring terminal protection elements that are electrically connected to a common ground support
122
(FIG.
1
B). Each set of protection elements includes a gas tube
124
, a pair of opposed end caps
126
, a fusible solder pellet
127
, an MOV
128
, and a failsafe MOV spring
129
having a first end
131
and a second end
133
. An elastic retaining band
130
holds the failsafe MOV spring
129
in position apart from the ground support
122
, thereby preventing a short-circuit between the line terminal
114
and the ground terminal
120
through the failsafe MOV spring
129
, the ground support
122
, and the ground cap
119
. The gas tube assemblies
118
are electrically connected between the voluted springs
116
and the ground cap
119
, which in turn is electrically connected to the ground terminal
120
. Thus, each conductor secured on a tip or ring line terminal
114
is electrically connected to the ground terminal
120
through one of the voluted springs
116
, the corresponding gas tube assembly
118
, the ground support
122
, the ground cap
119
, and the ground terminal
120
.
The gas tube
124
is disposed between the ground support
122
and the failsafe MOV spring
129
. The gas tube
124
has a first electrode
132
electrically connected to the ground support
122
, and a second electrode
134
spaced from the first electrode that is electrically connected to the first end
131
of the failsafe MOV spring
129
adjacent the voluted spring
116
. Accordingly, the surge protector
110
provides a first electrical ground path from the line terminal
114
, through the voluted spring
116
, through the first end
131
of the failsafe MOV spring
129
, between the second electrode
134
and the first electrode
132
of the gas tube
124
, through the ground support
122
, through the ground cap
119
, and out to the ground terminal
120
.
The fusible solder pellet
127
and the MOV
128
are disposed between the opposed end caps
126
. One of the end caps
126
is electrically connected to the ground support
122
while the other end cap
126
is electrically connected to the second end
133
of the failsafe MOV spring
129
. Accordingly, the surge protector
110
provides a second electrical ground path from the line terminal
114
, through the voluted spring
116
, between the first end
131
and the second end
133
of the failsafe MOV spring
129
, through the MOV
128
and the fusible solder pellet
127
between the opposed end caps
126
, through the ground support
122
, through the ground cap
119
, and out to the ground terminal
120
. The second electrical ground path is parallel to the first electrical ground path and diverts voltage surges to ground if the gas tube
124
fails (i.e., when the MOV
128
acts as a back-up protection device) or when the MOV
128
operates in conjunction with the gas tube
124
as a hybrid protection device.
In the event of a sustained voltage surge, the gas tube
124
and/or the MOV
128
will overheat, and thereby cause the fusible solder pellet
127
to melt. Once the fusible solder pellet
127
melts, the failsafe MOV spring
129
forces the outer edges of the opposed end caps
126
into contact with one another. Accordingly, the surge protector
110
provides an electrical short-circuit path parallel to both the first electrical ground path and the second electrical grou

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