CT Distance limits for 1ma DC to meter

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Juicenewt

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Ithaca,Nebraska,USA
This question has two parts;
Need to mount a ammeter with a 1ma full deflection approx. 70 ft. from the switch gear. Process Measurement Company told me once the 5ma from a CT is converted to 1 ma DC with the transducer I have no restrictions on the distance from the point of measurement. Has anyone used a CT with this small of a current for a distance of at least 70 ft.?
I intend to use shielded #18 twisted pair for the full length with the shield grounded at both ends and I was wondering if this will eliminate any interference from the main power conductors inside of the switch gear?
Any information appreciated.
 
gadfly56

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Juicenewt said:
This question has two parts;
Need to mount a ammeter with a 1ma full deflection approx. 70 ft. from the switch gear. Process Measurement Company told me once the 5ma from a CT is converted to 1 ma DC with the transducer I have no restrictions on the distance from the point of measurement. Has anyone used a CT with this small of a current for a distance of at least 70 ft.?
I intend to use shielded #18 twisted pair for the full length with the shield grounded at both ends and I was wondering if this will eliminate any interference from the main power conductors inside of the switch gear?
Any information appreciated.
Click to expand...

Do NOT ground at both ends. You may/almost certainly will wind up with ground loops.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
160914-1159 EDT

On a current loop your distance limit is defined by the maximum source voltage from the transmitter at 1 mA, and total loop resistance. A 1 mA DC meter is likely to beat 50 to 250 mV for full scale. Meter drop is probably insignificant in terms of the probable available source voltage.

If the maximum source voltage is 20 V, then total loop resistance could be up to near R = 20/0.001 = 20,000 ohms.

Unlikely any local interference would cause any problem with the DC meter reading. A DC meter might slightly flicker at 10 Hz. I doubt that shielding is needed, just twisted wire should be adequate. A shield will provide some protection for a direct short to the shield until the shield burns up.

.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
160914-1402 EDT

In my previous post "beat 50" is supposed to be "be at 50". Problem is junky space bar on Apple computer.

A Simpson 270 tautband (same as 260) meter on 1 mA DC scale with a 10 V peak sine wave current limited by a 10000 ohm resistor started to produce needle flicker at 14 Hz, and it had a peak swing of 0.42 mA at 0.2 Hz. With this meter there is a lot of filtering.

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I

Ingenieur

Senior Member
Location
Earth
Post the data sheet for the xdcr please
chances are it is rated to drive a hi R load so cable length is moot
it is a current source so i anywhere in the loop will be the same and the xdcr is basically an amp and will drive almost any load
 
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