Hi Dagmar,

It would be helpful if you could supply more information, including what yo=
u mean by=20
"stop working". There are hundreds of things you can do to protect circuitr=
y in=20
adverse conditions, but we don't know your circuit and we don't know which =
part=20
"stops" (i.e. could be just a glitch in data comms and a reset fixes it, or=
 firmware=20
bug, or firmware gets corrupted, or 5V PSU behaves badly in certain scenari=
os, or=20
permanent electrical damage occurs to micro/comms/pressure sensor, or...the=
 list=20
continues).

Sure, induced electrical currents are a likely factor given the relatively =
long lengths=20
of parallel wiring and high motor currents. And yes Varistors and TVS Diode=
s can=20
be very effective. But to have any degree of confidence you need to enginee=
r the=20
solution as accurately as possible against some known adverse conditions. F=
rom=20
memory RS485 devices will want the data lines clamped within -7 to +12V.

You say 5V supply, I hope that is not running your PIC VDD directly? Would =
need=20
extreme protection and filtering to make that even a slightly viable option=
.. Even with=20
a linear reg to 3.3V you don't have enough head room for the likely amount =
of hash=20
on the power supply. But that's just a guess.

Sounds like a fun project, well at least it should be fun once you've irone=
d out the=20
problems.

Brent.

On 21 Sep 2015 at 10:12, Dagmar Carnier Neto wrote:

>                  Hello all,
>                  I'm developing a digital water-level probe to be=20
> installed inside wells, and last week found some of them stop working.=20
> They were installed inside deep wells, around 250 meters deep. A 4-wire=20
> cable connect them to the top, being 2-wires for power (5V and GND) and=20
> two for communication (A and B, for a RS-485 half-duplex transceiver).=20
> The board inside the  probe is potted with resin, to avoid water=20
> intrusion inside the case, and not under high pressure (a few meters),=20
> so I discarded humidity problems.
>                  My best hypothesis is they stop working due induced=20
> surges. The cable is running inside an inspection tube (metal), that=20
> runs parallel to the power cables for the pump inside the well. The=20
> pumps are some heavy monsters, ranging from 250 HP to 450 HP. I don=B4t=20
> know how good is the local power-lines quality, usually here in Brazil=20
> they=B4re not very stable.
>                   Some clues:
>                   1-)Some of pumps burned out (more than once) and were=20
> replaced. There=B4s a range of reasons for that happening, since=20
> water-related reasons to electrical reasons, and I don't know which=20
> caused that.
>                  2-) The only equipment still working is inside a well=20
> with the lowest power pump (around 250 HP).
>=20
>                  I=B4m not an expert in high-power systems (for me,=20
> anything over 12V is high-voltage and over 500mA is a short-circuit=20
> current ;) so I=B4m asking for any helpful hint or advice. I'm considerin=
g=20
> using varistors to protect the lines, and grounding the cable shield to=20
> the well metallic case. What do you think? Or should I use TVS? I can=B4t=
=20
> see much difference between them...
>=20
>                          Thanks for the help, best regards,
>=20
>                                          Dagmar
>=20
> ---
> Este email foi escaneado pelo Avast antiv=EDrus.
> http://www.avast.com
>=20
> --=20
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> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist




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