Disconnecting Ammeters from Current Transformers

Well short-circuiting the CT to prevent accidental open-circuiting of the CT output would be a good start.

Switch 'OFF" the power supply before you disconnect the ammeter or short circuit the current transformer.

Don't dismantle out the ammeter with the current transformer loaded other wise your CT will burnt down.

<...Don't dismantle out the ammeter with the current transformer loaded other wise your CT will burnt down....>

This assumes that the individual disconnecting it hasn't died from electric shock first!

What are the steps that would take before disconnecting ammeter permanently supplied from a current transformer?

I would suggest 1) switch "off" the power supply 2) install terminal block suitable to the size of CT wiring 3) disconnect the two wires from the CT and re-connect it to the new terminal block 4) install jumper wire across these two CT wiring to short it out, or you can also connect the two CT wires in a single terminal in the terminal block 5) or you can connect jumper wire across CT secondary terminals. NOTE: make sure CT is not open circuited when power is "on", otherwise, high voltage will be present in the CT.

regards.

rudy c. fernando

roshanrrd; note: your removed AC ammeter would typically be 0-5 amps full scale. perry

I'm surprised that no one has asked, "Why?"

<Sarcasm>

Because the feeder being fed from the source that the CT/meter is watching is overloaded and if we remove the meter the problem goes away.

</Sarcasm>

Maintenance on a powered line perhaps? It is safe to play with the output of a live CT if you cannot wait until the power can be shut off (such as a substation CT), as long as you take adequate precautions.

sometimes ammeter should be removed with out switching off the power supply.in that case short circuit the secondary of ct.there will normally be provision for that.short circuiting the ct secondary is not a great problem as ammeter itself is offering only a very low resistance path.

then disconnect and remove the ammeter.

Roshanrrd,

As others have said a number of times, short the two CT secondary leads together. I know that in electrical work one normally thinks of a short circuit as a thing to be avoided. But here is the reason:

• The CT can be thought of a power supply capable of providing as much voltage (or as little voltage) as is needed to drive a given current through its output load. Thus it is current limiting, but not voltage limiting.
• Its output current is proportional to the current flowing through the conductor it surrounds.
• In the most common applications its secondary current will be in the range of 0-5A.

Now, if the load resistance is 5 ohms, the power it is producing is as much as 25watts. As you decrease the load resistance towards 0 ohms, the voltage across the terminals will decrease towards 0 because it is inherently current-limiting. If, on the other hand, you open the circuit, it will output a potential across the terminals that can approach the insulation breakdown voltage of the wires in the CT, and/or damage the CT in other ways so it no longer can provide an accurate proportional output when it is reconnected. So, yes, short its two terminals together before you disconnect any load meter, always. There usually will be jumpers on the terminal strip to allow this to be done without disconnecting any leads. Leave the shorting jumper in place until after the load meter is connected, always, even if it is on a de-energized circuit.

--JMM

Best way is to switch of power first, then disconnect and shortcircuit the trafo before you switch on power again.

It is essential to short circuit the trafo, If you can do it without disconnecting first than it is ok, fex. if there are two connection screws on each end on the trafo

Dear Mr.Roshanrrd,

You can use a test block installed before you open the terminals of tha Ammeter so that it is safe. Or else the voltage will be multiplied by the ratio of the Current transformer, leading to explosion or burning out of Current Transformer, or cause severe shock to the staff.

DHAYANANDHAN.S,

CR4 MEMBER,

INDIA.