Sharing arrangement for surge protection circuitry

Transient voltage surge protection circuitry for a plurality of transmission line-pairs (10-18) utilizes a single multi-terminal protective network (34) coupled to individual lines (52-56) of the transmission line-pairs by full-wave diode bridge circuits (48) and resettable fuse elements (62,64).

BACKGROUND OF THE INVENTION 
This invention relates to transient voltage surge protection circuitry for 
a plurality of transmission line-pairs carried by a multi-conductor cable 
and, more particularly, to an arrangement for sharing a single protective 
network among the plurality of line-pairs. 
Telecommunications transmission lines are typically susceptible to 
lightning strikes when not within a building. Accordingly, primary 
lightning protection is usually provided where the transmission lines 
enter a building. However, modern telecommunications equipment, such as 
computer controlled private branch exchanges, are very sensitive to 
transient surges such as those caused by lightning strikes and the primary 
lightning protection is often inadequate. Telecommunications equipment 
manufacturers therefore commonly provide secondary surge protection within 
their equipment. Such equipment, for example, a computer controlled 
private branch exchange, may have twenty, fifty or even more, telephone 
line-pairs connected thereto. Therefore, if the secondary surge protection 
is incorporated within the equipment, this leads to a number of 
disadvantages. One disadvantage is that the secondary surge protection 
takes up valuable "real estate" within the telecommunications equipment. 
Another disadvantage is that bulky cables carrying thick wire have to be 
utilized between the primary protection and the secondary protection. 
Co-pending U.S. patent application Ser. No. 08/515,181 proposes to 
overcome the aforementioned disadvantages by providing the secondary 
protection within an assembly connected between two multi-conductor 
cables. As disclosed therein, the protection circuitry comprises a 
respective protective network for each of the transmission line-pairs. A 
multiconductor cable, therefore, requires as many protective network chips 
as there are conductors. A large number of the protective network chips 
add bulk and cost to the secondary protection adapter to be interposed 
between two connectors in a telecommunication multiple line-pair system. 
The cost of the duplicated part becomes a significant portion of the 
overall manufacturing cost of the secondary protection. It would be 
desirable for each line-pair to share a single protective network. A 
sharing arrangement would reduce the size and cost of the secondary 
protection adapter. 
It is an object of the present invention to provide a circuit design which 
provides cost savings advantages over the prior design which uses a 
respective protective network for each of the line-pairs by permitting a 
single protective network to be shared among a plurality of line-pairs. 
SUMMARY OF THE INVENTION 
The foregoing and additional objects are attained in accordance with the 
principles of this invention by providing a circuit arrangement which 
utilizes a single multi-terminal protective network to provide transient 
voltage surge protection for a plurality of transmission line-pairs. 
A circuit arrangment comprises a telecommunications line pair connected to 
first and second terminals of a protective network. A first line in a 
plurality of the line pairs is connected to a first common terminal 
through a means for isolating the respective lines. The first common 
terminal is connected to the first terminal of the protective network 
through a resettable fuse element. 
It is an advantage of the present invention that a plurality of 
transmission lines can share a single protective network.

DETAILED DESCRIPTION 
Referring now to the drawing, a plurality of transmission line-pairs 10, 
12, 14, 16 and 18 connected to primary lightning protection (not shown) 
are coupled through secondary transient voltage surge protection adaptor 
20 to respective ones of the plurality of transmission line-pairs 22, 24, 
26, 28 and 30 which extend to telecommunications equipment (not shown). 
The transmission line-pairs 10-18 are preferably contained as part of a 
first multi-conductor cable (not shown), and the transmission line-pairs 
22-30 are preferably contained as part of a second multi-conductor cable 
(not shown). These multi-conductor cables are preferably coupled to the 
adaptor 20 by respective cable connectors (not shown). Although five sets 
of transmission line-pairs are illustrated, it is understood that more or 
fewer sets of line-pairs can be connected to the adaptor 20, which can be 
expanded or contracted as desired. 
The adaptor 20 includes a plurality of resettable fuse elements 32 each 
interposed serially in a respective line connecting the line-pairs 10-18 
with the respective line pairs 22-30. Illustratively, the fuse elements 32 
are of the type TR-600-150 sold by Raychem Corp. Such fuse elements open 
the electrical circuit therethrough when heated due to a threshold 
current. The fuse elements reset to short the electrical circuit when they 
cool down in the presence of a current below the threshold current. 
The adaptor 20 includes a single multi-terminal protective network 34 for 
providing transient voltage surge protection for all of the transmission 
line-pairs 10-18 and 22-30. The protective network 34 includes three 
SIDACtor elements 36, 38 and 40 connected in a "Y" configuration between a 
first terminal 42 of the network 34, a second terminal 44 of the network 
34 and a third terminal 46 of the network 34. The terminal 46 is connected 
to ground. The network 34 is illustratively of the type manufactured by 
Teccor Electronics, Inc. of Irving, Tex. and is fully described in U.S. 
Pat. No. 4,905,119. Thus, the protective network 34 includes a first 
bi-directional voltage sensitive switch 36 having first and second leads, 
a second bi-directional voltage sensitive switch 38 having third and 
fourth leads, a third bi-directional voltage sensitive switch 40 having 
fifth and sixth leads, means for connecting the first lead of the switch 
36 to the terminal 42, means for connecting together the second lead of 
the first switch 36, the third lead of the second switch 38 and the fifth 
lead of the third switch 40, means for connecting the fourth lead of the 
second switch 38 to the terminal 44 and means for connecting the sixth 
lead of the third switch 40 to the terminal 46. 
For coupling the line-pairs 10-18 and 22-30 to the protective network 34, 
there is provided a plurality of unidirectional current paths from each 
line 52,56 of each of the line-pairs 10-18 to a first common terminal 41 
and a plurality of unidirectional current paths from a second common 
terminal 43 to each line 52,56 of each of the line-pairs. Illustratively, 
these unidirectional current paths are provided by a plurality of 
full-wave diode bridge circuits 48, one for each set of line-pairs 10-18. 
Thus, for the set of line-pairs 10, 22, the diode bridge circuit 48-1 
includes a diode 50 for providing a unidirectional current path from the 
line 52 to the first common terminal 41; a diode 54 for providing a 
unidirectional current path from the line 56 to the first common terminal 
41; a diode 58 for providing a unidirectional current path from the second 
common terminal 43 to the line 52; and a diode 60 for providing a 
unidirectional current path from the second common terminal 43 to the line 
56. 
A first resettable fuse element 62 is connected between the first terminal 
42 of the protective network 34 and the first common terminal 41. A second 
resettable fuse element 64 is connected between the second terminal 44 of 
the protective network 34 and the second common terminal 43. In a 
preferred embodiment the fuse element 62,64 are of the same type as the 
fuse elements 32. The fuse elements 62,64 perform the following function; 
in response to a voltage surge due to, for example, a lighting strike, the 
SIDACtors 40 and 36 or 38 are turned on, shorting the voltage surge to 
ground potential. When the voltage surge passes and the line pairs 10-18, 
22-30 return to their normal operating conditions, the leakage currents 
from all of the line pairs 10-18 and through the diodes 50,54,58,60 when 
summed at the first or second common terminal 41,43 will, in the absence 
of resettable fuse elements 62,64, not permit the SIDACtors 40 and 36 or 
38 to turn off to open the circuit between the line pairs 10-18 and ground 
46. The additive current from all line pairs 10-18 is above the current 
turn off threshold of the SIDACtors in the protective network 34. The fuse 
element 62,64 provide a current responsive element to provide an open 
circuit in response to the additive current from the line pairs 10-18. 
When the voltage surge has passed, therefore, the additive current which 
is a sum of the leakage currents of the line pairs 10-18 in their normal 
operating condition will cause the resettable fuse element 62 or 64 to 
open. Opening of the circuit that provides a current flow through the 
SIDACtor elements permits the SIDACtor elements to reset to their normally 
off condition that provides an open circuit to ground potential in the 
absence of a threshold voltage. After the SIDACtor elements 38,38,40 of 
the protective network 34 have provided a halt to the current flow through 
resettable fuse element 62 or 64, the resettable fuse elements 62 or 64 
cool down in the absence of the current flow and turn back to their normal 
state of a short circuit. The resettable fuse element and SIDACtor circuit 
therefore returns to its normal operating state which is a short circuit 
connection between the first common terminal 41 and first terminal 42 as 
well as a short circuit connection between second common terminal 43 and 
second terminal 44. The short circuit connection through resettable fuses 
62 and 64 provide for a sufficiently direct connection between line pairs 
10-18 and protective network 34 so that protective network 34 is 
sufficiently responsive to voltage surges on line pairs 10-18. The 
resettable voltage responsive protective network 34 in combination with 
resettable fuse elements 62,64 therefore provide for a resettable and 
voltage responsive protective circuit that may be shared among a plurality 
of line pairs. 
Line pairs 10-18 are connected to first and second common terminals through 
a full wave diode bridge circuit 48. The diode bridge circuit provides a 
bi-directional current path to the protective network 34 through first and 
second common terminals 41,43 as well as voltage isolation between lines 
52,56 of respective line-pairs 10-18. Further, the full-wave diode bridge 
circuits 48 connected in series between the respective transmission 
line-pairs and the protective network 34 results in a lowering of the 
overall capacitance of each of the line-pairs. This improves the circuit 
high frequency performance. 
If a positive transient voltage surge appears on the line 52, the current 
accompanying such surge passes to the protective network 34 through the 
diode 50. If a positive transient voltage surge appears on the line 56, 
the current accompanying such surge passes to the protective network 34 
through the diode 54. If a negative transient voltage surge appears on the 
line 52, the current accompanying such surge is carried by the diode 58. 
If a negative transient voltage surge appears on the line 56, the current 
accompanying such surge is carried by the diode 60. Accordingly, by 
providing the resettable fuses 62 and the diode bridge circuits 48, a 
single protective network 34 can be utilized for all of the transmission 
line-pairs in place of one protective network 34 for each of the 
line-pairs. Although the diode bridge circuits 48 are provided (one for 
each set of line-pairs), due to the relative costs of the components, it 
has been found that the circuitry shown in the drawing and described above 
results in significant size and cost saving for an adaptor 20 which 
handles twenty-five transmission line-pairs over the cost of an adaptor 
with a protective network for each of the line-pairs. 
Accordingly, there has been disclosed improved circuitry for providing 
transient voltage surge protection for a plurality of transmission 
line-pairs. While an illustrative embodiment of the present invention has 
been disclosed herein, it is understood that various modifications and 
adaptations to the disclosed embodiment will be apparent to those of 
ordinary skill in the art and it is intended that this invention be 
limited only by the scope of the appended claims.