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Ammeter switch connection

12/28/2021 3:39 PM

Hello everyone! I have problem with ammeter switch from attachment. Here is connection diagram, and everything looks fine. But, if we look carefully, we will see that ground and L3 are short connected via 3-4 terminals, when we put switch in L1 position. So, my question is, how it's possible, it looks like short-circuit?

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#1

Re: Ammeter switch connection

12/28/2021 4:15 PM

The ammeter is not connected to the line phase power, it is supplied by current transformers....

https://engineering.electrical-equipment.org/others/measuring-current-with-ammeters.html

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#2
In reply to #1

Re: Ammeter switch connection

12/28/2021 5:10 PM

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#3

Re: Ammeter switch connection

12/29/2021 1:02 AM

As anyone who deals with current transformers, one must be aware that the ammeter connects across one CT secondary at a time while the unmetered CTs must be shorted by the switch connecting the ammeter. The switch used must be a make before break type so that at all times the CT secondary must be either shorted by a shunt or an ammeter.

If not then suppose that the primary is at 415 volts and the CT secondary is 100:1 then it could be possible to generate 41.5KV. Of course the CTs do have impedance limitations imposed on them by design. Once saw a 600:1 CT open circuited with only a few volts impressed across the primary to develop 600 amps in the primary during a test and the arc drawn was very significant and shocking to the person who disconnected the test clip by mistake.

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#5
In reply to #3

Re: Ammeter switch connection

12/29/2021 10:15 AM

Stef is correct. All sorts of "bad things" can happen if the secondary winding of a CT (current transformer) is left open when current is flowing through the primary. Shorting out the secondary of a CT is safe because there is only enough voltage on the secondary to make the current flow according to the CT's ratio, such as 100:5 or many others. The ammeter's movement (or electronics) form a very low resistance circuit--take one and use a good meter to read its resistance.

Your question is understandable, because many electricians are unfamiliar with CT's and this requirement for their safe operation. Indeed, the manufacturers' literature will usually also tell you the maximum resistance permitted on the secondary circuit in order for the CT to be reading accurately.

JMM

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#4

Re: Ammeter switch connection

12/29/2021 2:32 AM

I think use of switch in Ammeter circuit is not recommended for the fear of CT open circuit.

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#6

Re: Ammeter switch connection

12/29/2021 3:11 PM

One must always ensure that current transformers [CT] never go open circuit. In the case illustrated, the switching ensures that only one CT is connected to the ammeter at any time, and any not being monitored are shorted.

The switching in the diagram is, therefore, correct.

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