I can't understand this meter

[ali999]

Hi

I need help understanding this ammeter that I recently came across in a hospital. See attached picture. The unit is old (mid 70's) so it is almost impossible to find any useful info online. The meter is connected to a main switchboard rated at 1600A. As far as I understand, the white needle indicates what the max allowed current is while the red needle indicates what the actual reading is. So if the red needle crosses the white needle some sort of alarm probably goes off and a breaker trips. However, what are the smaller numbers on the gauge supposed to indicate? These are all exactly 1/2 of the larger ones. This probably has something to do with the amp=3/6 but I don't understand what that means either. Also, what does the smaller meter indicate? It says AC amperes on it but I know that all the current in this switchboard is AC. The smaller needle also occasionally has small movements while the larger ones remain stationary. Anyone here come across something like this before? Any help would be greatly appreciated.

 

[Skogsgurra]

First, the 3/6. That is probably the secondary current of the transformer connected to the meter and shown on the small instrument (which probably has a soft-iron movement).

The two pointers (red and white) are probably thermal and react very slowly to the current. Such movements are usually bi-metal type with a heating winding. Very much like thermal protection in classical motor starters.

Such movements usually have a rather long time constant. Fifteen or thirty minutes are common. That corresponds to thermal time constant in many components like small generators and transformers and traces the temperature of those components in a rather crude way.

It is very probable that the white pointer is nothing but a peak indicator that is being dragged upscale by the red pointer to indicate what the maximum thermal load has been since last reset. I think that Bill (Ross, that is) has a very good idea about this instrument.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[alehman]

guess: Thermal averaging amp demand meter with two input scales 3A and 6A. Red is actual, white is maximum. The small meter within could be a normal response meter. Maybe you turn the little lever on the front to reset the demand pointer.

http://www.hdelectriccompany.com/assets/files/AD7.pdf

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[waross]

It is very probable that the white pointer is nothing but a peak indicator that is being dragged upscale by the red pointer to indicate what the maximum thermal load has been since last reset. I think that Bill (Ross, that is) has a very good idea about this instrument.

I agree Gunnar. If we are correct, the hardware on the front of the meter (looks like stainless steel) will lift and turn to reset the demand needle (white needle).
Follow alehman's link for more information.
The needles on an alarm meter look similar, but there would be an obvious means to set the alarm pointer if this was an alarm meter.
As for the two scales and the 3/6 amp note; This is common on meters for dual voltage generators.
I agree with Skogsgurra's and alehmen's comments.

Bill
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"Why not the best?"
Jimmy Carter

 

[Morrand]

I agree, this looks very, very much like the thermal ammeters that we once installed in substations for feeder load monitoring, and (minus the instantaneous meter) that we still sometimes install for transformer monitoring. Oddly, we still have ours catalogued internally as a Sangamo Electric product some 35 years after their demise: in fact, we're getting them from HD Electric now, which lists both our ADS-7 and an ADF model on their Web site:

http://www.hdelectriccompany.com/hd...g/ampere-demand-meters/AD-Series-Ammeters.htm

Notice the little wire coming across near the letters "ADF" on the meter face: that's your maximum demand pointer pusher, connected to the reset lever at bottom center.

 

[PHovnanian]

It appears to be a combination thermal/instantaneous ammeter with a resettable peak indication (the white hand). One can reset the peak indicator with the little lever on the front (with the OK of the appropriate personnel). Lift the bottom of the lever and rotate CCW until the white hand reaches the red one. Return it to its present position. It can be lockwired to prevent inadvertent tampering if needed.