Maker Pro
Maker Pro

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

Hi all!

For my grandpa's country I am going to install tens of 4-20mA sensors,
which will be routed through some hundreds meters of good cable to the
data logger, at the border of the house. To give some (yes, I know it's
impossible to give total) protection against nearby lightings, etc.. I
thought about adding surge protection.

If I understand it right, I should put one device in parallel with each
sensor, and another device in parallel with each data logger input (i.e.
the two ends of each cable), right?

Varistors, gas-dischargers, etc.. to me it seems that a Transil diode may
be a very cheap solution but a very effective one nonetheless. Am I wrong?

And, being there two in parallel for each cable (one at the sensor and
the other at the data logger input), are they going to false the results
by much? I am aiming at 16bit resolution, with a full scale precision of
about 0.1%

Thank you!
Andrea
 
S

Steve Y

Jan 1, 1970
0
If you want to protect the datalogger and the sensor then you will need
protection at both ends. Just using diodes will not give you sufficient
protection, you will need gas discharge tubes as well.

If you want to protect the sensor and the electronics are not earthed, a
parallel connected device will be OK but for the datalogger you will
better with a series connected device, this will give you more
protection and react quicker.

Any true industrial product will not introduce any errors into your
signal, especially as you are using 4/20mA signals rather than voltage

Steve
www.mtlsurge.com
 
W

w_tom

Jan 1, 1970
0
Same protection that permits your telephone CO to operate without
damage during every thunderstorm is also your solution. Remember the
difference between differential and longitudinal mode currents? You
are installing differential mode protection. But destructive surges
are longitudinal mode. A surge voltage is same on both wires
(overhead or underground) as current passes destructively through
datalogger to earth ground. What would a zener diode between wires
see? Zero voltage as thousands of volts confront a datalogger.

How does a telco everywhere operate during every thunderstorm
without damage? Bennison, Ghazi, and Ferland measured surges on
telephone wires during thunderstorms in 1968 and 1969 in IEEE
Transactions on Communications. Hundreds of transients occurred in
each thunderstorm. Yet damage is unacceptable. Protection during
thunderstorms is that routine in every day in every town even 70 years
ago.

An application note from one industry professional demonstrates the
technique:
http://www.erico.com/public/library/fep/technotes/tncr002.pdf
Each wire that enters a structure must connect to a single point earth
ground - either directly or via a protector. In your case, a
protector may be a transzorb, gas discharge tube, or MOV. Protector
is not protection. Protector is a connecting device to protection.
What did Bennison et al demonstrate? Destructive transient is
longitudinal. It seeks earth ground either via a data logger OR safety
earthed before entering a building. Protector must dump a surge into
earth long before it can get to the datalogger.

Essential to protection is single point earth ground. Incoming
wires inside every cable must connect to that same earthing electrode
at the service entrance. No earth ground means no effective
protection.
 
J

Jake Brodsky

Jan 1, 1970
0
Hi all!

For my grandpa's country I am going to install tens of 4-20mA sensors,
which will be routed through some hundreds meters of good cable to the
data logger, at the border of the house. To give some (yes, I know it's
impossible to give total) protection against nearby lightings, etc.. I
thought about adding surge protection.

If I understand it right, I should put one device in parallel with each
sensor, and another device in parallel with each data logger input (i.e.
the two ends of each cable), right?

Varistors, gas-dischargers, etc.. to me it seems that a Transil diode may
be a very cheap solution but a very effective one nonetheless. Am I wrong?

And, being there two in parallel for each cable (one at the sensor and
the other at the data logger input), are they going to false the results
by much? I am aiming at 16bit resolution, with a full scale precision of
about 0.1%

Thank you!
Andrea
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.

The problem you get in to with long runs of 4-20 mA current loops is
that the local ground potential at each end can be very different. If
you put a surge protector at each end, then one or both are almost
guaranteed to fire with any nearby lightning strike.

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.

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.

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...

Jacob Brodsky, PE
 
P

Paul E. Bennett

Jan 1, 1970
0
Jake said:
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.

--
********************************************************************
Paul E. Bennett ....................<email://[email protected]>
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..
********************************************************************
 
Top