On 5/18/99 Dwight Hunnicutt writes:
>Question regarding "two-legged" vs. "three-legged" overvoltage
>protection circuitry: What are the pro & cons of the two?
>Background: Typically, to protect against overvoltages on a telco
>interface protection circuit (analog or digital, such as POTS, T1, HDSL,
>etc.), one sees either:
>1) an MOV/varistor type device across TIP/RING, or
>2) two MOV/varistor/gas-tube type devices tied in series across
>TIP/RING, with the center connection tied to earth ground.
>Of course, there are also typically PTC's or fuses in line for
>overcurrent protection. However, my interest is the pros/cons of the
>overvoltage protection topology.
>If the interface circuit has no path to earth (typically through
>overvoltage protectors), then UL1950/UL1459 allows waiving of the
>longitudinal(common) mode overvoltage tests, which makes sense, because
>there is no return path for the fault energy.
>Since this waiver eliminates about half of the overvoltage testing, why
>does one see the "three-legged" topology being used? Are there some
>advantages to shunting energy to earth, rather than just back out the
>TIP/RING pair? Certainly, one has to provide overcurrent protection to
>prevent building telco wiring from burning (tested via the MDQ 1-6/10A
>fuse), but are there other reasons for preferring a three-legged
>approach? What are you missing out on if you elect to use the simpler
>topology of just an MOV across TIP/RING?
>(To further stir things up, how about if we take into consideration
>Bellcore GR-1089-CORE? Does that change things? I don't believe
>GR-1089 specifically contains the same waiver as UL1950/UL1459, but
>certainly the results are the same, and a test lab should consider
>waiving for the same rationale.)
>I'm sure many of you have seen both topologies described in application
>notes for various interface components, and have had to deal with both.
>Any light shed will be appreciated by all.
There are steady-state overvoltage conditions (power cross/induction)
and impulse conditions (lightning/ESD). Protection for one does not
necessarily do both.
If the T/R pair is truly floating, then yes, longitudinal protection
is questionable. Many circuits, especially those in a telephone
central office, do not float. There are ground start (not as many
now) and battery feed circuits. The feed could could be either across
T/R or simplexed across the T/R and T1/R1 pairs. These cicuits,
despite secondary protection on the equipment side of the transformer,
still need clamping to 230 Vac, 300 Vac, or whatever. Some circuits
don't even have transformers; they come right off the SLIC. Makes OV
protection and longitudinal balance design real interesting.
So, "How well does the circuit float?" (for impulse AND steady-state)
is the big question....
Eric Petitpierre
Pulse Communications
Herndon, VA
eric.petitpie...@pulse.com
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