Mysterious relay in water telemtery panel / SCADA
Mysterious relay in water telemtery panel / SCADA
(OP)
Hi folks,
I recently was tasked with replacing some old water system telemetry panels with new SCADA gear. I did so, but there was a mysterious relay in one of the old panels. It cycled on/off every two seconds. Since I was replacing everything, I never found out what this relay did.
Recently I started upgrading a second water system. I found a relay in this panel too, same 2-second cycle time.
This panel measured only two 4-20ma levels from two water tanks. I pulled the relay and the loop current fell to zero. Anyone have an idea what this relay's purpose might be? Thanks in advance,
-
Robert
I recently was tasked with replacing some old water system telemetry panels with new SCADA gear. I did so, but there was a mysterious relay in one of the old panels. It cycled on/off every two seconds. Since I was replacing everything, I never found out what this relay did.
Recently I started upgrading a second water system. I found a relay in this panel too, same 2-second cycle time.
This panel measured only two 4-20ma levels from two water tanks. I pulled the relay and the loop current fell to zero. Anyone have an idea what this relay's purpose might be? Thanks in advance,
-
Robert





RE: Mysterious relay in water telemtery panel / SCADA
The reason I ask is that the underground infrastructure around these parts has the usual cute Yagi antennas pointed off in odd directions, but they also sometimes have an orange light bulb on a pole that blinks when there's trouble.
RE: Mysterious relay in water telemtery panel / SCADA
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Mysterious relay in water telemtery panel / SCADA
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Mysterious relay in water telemtery panel / SCADA
The system that has two 4-20s has two completely different set points, so I think this relay must do something else.
Maybe I'll just have to trace it out.
The other day I was working on one of their sytems that had a phase failure relay in it. I found that the original installer had wired it out of sequence, so to get it to work, he used the normally closed contacts. So it would have worked to protect against phase sequence reversal, but not phase loss. It had been wired that way for 25 years.
RE: Mysterious relay in water telemtery panel / SCADA
By the way, Sequim Washington?
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Mysterious relay in water telemtery panel / SCADA
Some legacy process-control systems sometimes had a 'watchdog' timer or 'health-check' feature. I wonder if it could be something like that?
RE: Mysterious relay in water telemtery panel / SCADA
Yes, I am in Sequim, WA.
http://solar.nikola.com
-Robert
RE: Mysterious relay in water telemtery panel / SCADA
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Mysterious relay in water telemtery panel / SCADA
Keith Cress
kcress - http://www.flaminsystems.com
RE: Mysterious relay in water telemtery panel / SCADA
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Mysterious relay in water telemtery panel / SCADA
Yes, the rainfall is only about 17" in Sequim, thanks to the rain shadow. A search for maximum sunlight on the peninsula for my planned solar trackers is what landed me here.
-Robert
RE: Mysterious relay in water telemtery panel / SCADA
There is a site (a prison) that this system pumps water to. It is this site that controls this relay coil. The contacts of this relay go to what I think is a retriggerable one-shot.
If the prison loses power to its telemetry, the one shot times out and disables the pumps.
Eventually I'll get a chance to trace everything. Tomorrow I get to visit the prison. Had to get a background clearance. Busbar: Can you believe it? I cleared the background check! The conviction for blowing up the bathroom 40 years ago has rolled off my record!
RE: Mysterious relay in water telemtery panel / SCADA
Time flies when you're having fun..
RE: Mysterious relay in water telemtery panel / SCADA
"Hey, come in here for a project... Oops, look what we found on your "Permanent Record"! Have a nice stay.".
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Mysterious relay in water telemtery panel / SCADA
seq—
It occurred to me that discussions of explosives on an open forum will put us both back on Government Watch Lists.
RE: Mysterious relay in water telemtery panel / SCADA
They took us on the complete tour. They have two 1-megawatt backup generators. Dentists, doctors on staff.
1000 prisoners, 400 staff.
Anyway, the relay: It's 4-20mA presented as PWM! 12 second period. 3 seconds on = 4mA. 12 seconds on = 20mA.
Nobody knows whey they chose to run a 108 volt DC pwm. Distance is 1 mile. The guy that did it isn't around.
RE: Mysterious relay in water telemtery panel / SCADA
RE: Mysterious relay in water telemtery panel / SCADA
Now days we would likely do it with hard line (FSK tone transmission, Ethernet, various industrial protocols) fiber optics (Ethernet etc.), or wireless communications, but PWM is still used in some industries and areas where the other methods are not feasible.
This is as close as I could find in a hurry as an aid to understanding the technology behind it.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Mysterious relay in water telemtery panel / SCADA
RE: Mysterious relay in water telemtery panel / SCADA
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Mysterious relay in water telemtery panel / SCADA
AGM (Arizona Gear and Machine) made (makes?) senders and receivers, solid state, that could be used to replace BIF equipment at one end or the other.
Voltages used for transmission were usually DC and two signals could occupy one pair of wire by using two power supplies, different polarities, and diodes at the receiving end.
AC could be used but a relay was required at the receiving end. I used tone and a tone receiver in cases where the TELCO phased out the metallic lines and only offered audio lines.
Steve
RE: Mysterious relay in water telemtery panel / SCADA
It's called a "Pulse Duration Transmitter" I put it on the bench and found:
1 volt in / 3 seconds out
5 volts in / 12 seconds out
So I programmed the PLC to measure the duration. Damn thing was accurate over its entire range of better than 1%, which is better than the 4-20mA isolaters that we were going to replace it with.
-Robert
RE: Mysterious relay in water telemtery panel / SCADA
Thanks for the followup, Robert. It's always nice to hear of resolution to sometimes agonizing troubleshooting effort.