Re: Surge protection for 4-20mA sensors and data logger?

["Paul E. Bennett" <peb@xxxxxxxxxxxxxxxxxx>]

Jake Brodsky wrote:

> There are two reasons to install protectors:  First, you need to protect
> the nearby structure from fire caused by the discharge.  Second, you
> might want to continue working through the discharge and maintain
> minimum noise pickup.  These goals often conflict with each other.
>
> The goal of fire safety is to limit the damage by grounding the cable
> shield frequently.  However, this can introduce quite a bit of noise.
> Some multi-conductor cables have two shields: One around the bundle, for
> grounding everywhere, and one around the pair for grounding locally.

Which is why there are two distinct names for the functions (Shield and
Screen). On things like Type 23 frigates the shield was grounded at each
and every bulkhead it passed through while the screens were grounded only
at the instrument rack ends. However, Grounding of Shields is a matter for
individual installation analysis to suit the environmental conditions
appertaining to the installation.

[%X]

> My suggestion is to use an isolated differential instrument.  Ground
> everything to ONLY ONE ground and put the protectors there.  Use
> whatever discharge devices you can afford. In my experience, what
> matters more is that these things get replaced right away.

What I use as standard. It precludes consideration of using equipment from
the likes of National Instruments for fast data aquasition in many of the
environments I deal with because of their lack of isolation and their
propensity to poor-man's differential input (use of two inputs combining to
provide the differential input).

> It's also important for you to use isolated analog inputs.  Having one
> input fire and not the others will result in a lot of damage unless the
> inputs are isolated.

Galvanically isolated, Energy Managing, Surge Clamped inputs and outputs for
everything is fairly normal practice for me (analogue and digital). In
short, attention to detail (such as ensuring enough terminals for each and
every conductor, decent signal earthing scheme, decent protective earthing
strategies, isolation strategies, termination management, design for safe
testing) is very important.

> People write books about this stuff.  This is just a very quick
> overview.  I suggest you read up on some telephone company grounding
> manuals.  They have experience with this sort of thing...

Not only the tel-co's but also the Oil and Gas Industry, and the Railway
Industry have large amounts of information on this topic.

--
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Paul E. Bennett ....................<email://peb@xxxxxxxxxxxxxxxxxx>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
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