- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Is ampmeter wiring from the CT's to the ampmeter required to be twisted? Or is it a good practice?
Ampmeter Wiring
- Thread starter tom baker
- Start date
- Status
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Ampmeter Wiring
No, it is not required. Yes it is a good practice internal to switchboards and other places with "open" wiring.
No, it is not required. Yes it is a good practice internal to switchboards and other places with "open" wiring.
Re: Ampmeter Wiring
One thing not to overlook is that an ammeter circuit should never be left as an open circuit if there is current flowing in the circuit being measured. I may be preaching to the choir but the voltage developed across an open circuit from a CT can be extremely high so you may want to also include a warning label attached to both the CT and the ammeter not to leave open circuit or to short the circuit with a jumper prior to removing the ammeter if required.
One thing not to overlook is that an ammeter circuit should never be left as an open circuit if there is current flowing in the circuit being measured. I may be preaching to the choir but the voltage developed across an open circuit from a CT can be extremely high so you may want to also include a warning label attached to both the CT and the ammeter not to leave open circuit or to short the circuit with a jumper prior to removing the ammeter if required.
Re: Ampmeter Wiring
Think of it this way, An ammeter is basically a shunt in itself, a short circuit so to speak. A CT generates current in its secondary winging in proportion to the current flowing in the primary.
The current can only flow in the secondary circuit in a specified proportion to that which is flowing in the primary if the secondary flow is unrestricted, that is that there is no resistance. Thus, if you were to try to measure the voltage in the secondary circuit which commonly includes and ammeter, you would read '0' volts.
Simply opening that circuit at any point, such as removing a lead from the CT or the Ammeter, will allow a voltage to develop across the open circuit when current is flowing through the primary of the CT, a voltage that could be quite dangerous. The question then would be how high would that voltage be? If one were to consider a 4:1 CT is it possible to induct 4x the primary voltage into the secondary, i.e. 480x4=1920v? Is it possible to exceed the common 600v rated insulation of the CT wire and cause if to fail?That's why it is imperative to short the wiring from the CT or the CT itself when servicing thew CY circuit and its components when there is current flowing through the circuit being monitored, this also includes protective relaying such as common CO relays.
Think of it this way, An ammeter is basically a shunt in itself, a short circuit so to speak. A CT generates current in its secondary winging in proportion to the current flowing in the primary.
The current can only flow in the secondary circuit in a specified proportion to that which is flowing in the primary if the secondary flow is unrestricted, that is that there is no resistance. Thus, if you were to try to measure the voltage in the secondary circuit which commonly includes and ammeter, you would read '0' volts.
Simply opening that circuit at any point, such as removing a lead from the CT or the Ammeter, will allow a voltage to develop across the open circuit when current is flowing through the primary of the CT, a voltage that could be quite dangerous. The question then would be how high would that voltage be? If one were to consider a 4:1 CT is it possible to induct 4x the primary voltage into the secondary, i.e. 480x4=1920v? Is it possible to exceed the common 600v rated insulation of the CT wire and cause if to fail?That's why it is imperative to short the wiring from the CT or the CT itself when servicing thew CY circuit and its components when there is current flowing through the circuit being monitored, this also includes protective relaying such as common CO relays.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Ampmeter Wiring
In theory at least, a current transformer will attempt to develop whatever secondary voltage is necessary to drive the secondary current(Lenz's Law); hence 110.23.
[ May 19, 2005, 12:59 PM: Message edited by: rbalex ]
In theory at least, a current transformer will attempt to develop whatever secondary voltage is necessary to drive the secondary current(Lenz's Law); hence 110.23.
[ May 19, 2005, 12:59 PM: Message edited by: rbalex ]
Re: Ampmeter Wiring
Thanks tempdl and rbalex,
I was thinking of it backwards. Yes indeed, according to my text, first copyrighted in 1924, the ammeter terminals should be shorted whenever the ammeter is not in use.
As I see it, an open secondary would place the primary inductance in series with the line. With the ammeter in place, its impedance would shunt the primary inductance.
Thanks tempdl and rbalex,
I was thinking of it backwards. Yes indeed, according to my text, first copyrighted in 1924, the ammeter terminals should be shorted whenever the ammeter is not in use.
As I see it, an open secondary would place the primary inductance in series with the line. With the ammeter in place, its impedance would shunt the primary inductance.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Re: Ampmeter Wiring
If I may offer an explanation that I've given to helpers and other interested peolpe:
A typical CT is available as 400:5 (equal to 80:1), meaning that one amp will flow through the meter for each 80 amps in the line (for a given phase, of course).
I've never opened one, but I take it that the CT has 80 turns of wire. Therefore, with an open secondary, the CT would operate as a 1:80 step-up transformer.
With even a lowly 120v primary, that would be (80*120) 9.6kv! That's higher than the average insulation rating, to say the least, as well as quite lethal.
If I may offer an explanation that I've given to helpers and other interested peolpe:
A typical CT is available as 400:5 (equal to 80:1), meaning that one amp will flow through the meter for each 80 amps in the line (for a given phase, of course).
I've never opened one, but I take it that the CT has 80 turns of wire. Therefore, with an open secondary, the CT would operate as a 1:80 step-up transformer.
With even a lowly 120v primary, that would be (80*120) 9.6kv! That's higher than the average insulation rating, to say the least, as well as quite lethal.
Re: Ampmeter Wiring
We have many, many CT's. From 50:5 up to 4000:5.
For new equipment, we specify short-circuiting-type terminal blocks.
We have even retrofit these blocks into existing equipment in some cases.
If, for whatever reason, you ever need to open the secondary wiring, this block is an easy place to short circuit the output of the CT secondary first. Most of them are set-up such that inserting thumbscrews & snugging down is all that is necessary.
I agree that twisted wiring is not needed, but is probably a good practice.
We have many, many CT's. From 50:5 up to 4000:5.
For new equipment, we specify short-circuiting-type terminal blocks.
We have even retrofit these blocks into existing equipment in some cases.
If, for whatever reason, you ever need to open the secondary wiring, this block is an easy place to short circuit the output of the CT secondary first. Most of them are set-up such that inserting thumbscrews & snugging down is all that is necessary.
I agree that twisted wiring is not needed, but is probably a good practice.
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