[Boatanchors] AC Meter Shunt Help]

[WA5CAB at cs.com]

Other comments not withstanding, a CT has inherent safety features relative 
to an AC ammeter with the line actually connected to it.  The typical 
implimentation is a one turn primary with the secondary determining the ratio.  The 
heavy line conductor is simply run through the center of the toroid without 
disturbing its insulation.  So to a first approximation it does not matter what 
voltage is on the line, it never appears on the meter, which may be anything up 
to several feet away from the CT.

In our 45 KW Onan, the CT's are about 2 inches in diameter so they readily 
handle the #1/0 cable used in our single phase installation.  Imagine if you 
will having to route this heavy cable (or four of them) up to the control panel 
and then back to the junction box.  And the force applied to the studs on the 
meters and to the door hinges when you open or close the doors on the control 
unit.

In a message dated 4/15/2007 3:08:48 PM Central Standard Time, 
brian.goldsmith at echo1.com.au writes: 
> 
> Personally, I would NOT use a CT!  They can DEFINITELY be dangerous!
> 
> ***** So can motor cars!!!
> 
> Take a look at VTVMs and DVMs.  They ALL use a hard wire SHUNT to 
> achieve their high amp readings.  That's how go from the normal range 
> steps, I.E., 200uA, 2000uA, 20mA, and 200mA reading scales to the 10A 
> position.
> 
> When placed into the 10A position, a solid but tinned copper shunt is 
> added to the circuit is placed in the circuit and the meter movements 
> ACTUAL full scale is read by the aqcross resistance of the shunt!
> 
> 
> *** Pity that the electrical industry does not agree with you,Current
> Transformers are universally used to allow heavy currents to be read on
> standard low current meter movements since the invention of the alternator
> and the transformer.
> DVMs are inherently DC operated devices,to read AC they require a rectifier
> arrangement which introduces severe non lineararities in the scale
> readings.As well,copper has a positive coefficient characteristic of its
> electrical resistance,as it heats, the accuracy of the meter reading is
> severely flawed.
> 
> Are you seriously suggesting that the millions and millions of CT operated
> meters and protection devices all over the world be replaced by rectifier
> and shunt type meter movements?
> 
> The mind boggles!!
> 

Robert Downs - Houston
<http://www.wa5cab.com> (Web Store)
MVPA 9480
<wa5cab at cs.com> (Primary email)
<wa5cab at houston.rr.com> (Backup email)