Surge protection as follows - per conductor of data, of which there are two being used, although eight circuits on the cards.
input from field
half amp fuse in series
spark gap to ground
avalanche diode to ground
toroid in series
MOV to ground
--- end of first biard
ORANGE DROP in series
--- then on the next board
input from first surge board
1.2ohm nichrome resistor
MOV to ground
.1amp fuse
MOV to ground
1.2ohm nichrome resistor
--- end of 2nd board
ORANGE DROP in series
--- then on next board
input from the second surge board
MOV to ground
toroid in series
avalanche diode to ground
spark gap to ground
half amp fuse in series
--- output to equipment
All brought to earth grounds with less then 5 ohms EGR, as measured with my calibrated Megger EGR tester
Normally just the middle board is used, but in this case, I added additional protection at the customers request. On his field units, he only has one of the "end boards" each, and these blew both half amp fuses, but no other damage.... for a total of about 70something fuses blown for one lightning storm. Type 3AG half amp fuses can be had for $0.18each when bought in 1000 units at Mouser. We get 10,000 at a time, and I'm not sure the price, maybe a penny or penny and a half cheaper.
This data line carries what is essentially a non biased carrier at about 1/2 volt level, so is not too easy to protect. I think we do a decent job of it.
dbw said:
Why not get a surge protector? And why use orange drops when a 5-cent electrolytic would work just as well? :icon_scratch:
This setup is about $50 per circuit in components (toroids are expensive), as mfg cost, and sells for well over $2000. The electrolytic would not conduct well in both directions, hence the non-polarized Orange Drop (known for robustness, and its 600v rating). The design is a classic back-to-back multistage protection scheme, designed to blow off excess to ground, and staged to give rise time preferences. The fuses are there to protect the rest of the system in the event of a constant short failure mode of one of the transorbs, movs or sparkgaps. Whatever hit, hit harder than 600v and hit longer than the heat could dissipate. Check out the vaporized dots of aluminum inside the core of the cap..... neat!