Ammeter switch connection

[Designmen90]

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?

 

[Julie.]

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?

Looks correct to me.

When it's
in o each ct is shorted direct to the star point,
In L1, L1 passes through the ammeter, L2, and L3 are shorted direct to the star point,
In L2, L2 passes through the ammeter, L1, and L3 are shorted direct to the star point,
In L3, L3 passes through the ammeter, L1, and L2 are shorted direct to the star point.

Which is what you want.

 

[Petej999]

Looks correct to me.

When it's
in o each ct is shorted direct to the star point,
In L1, L1 passes through the ammeter, L2, and L3 are shorted direct to the star point,
In L2, L2 passes through the ammeter, L1, and L3 are shorted direct to the star point,
In L3, L3 passes through the ammeter, L1, and L2 are shorted direct to the star point.

Which is what you want.

Agree

 

[Lucien Nunes]

The crucial point here being that the secondary of a current transformer should never be left open-circuit, as a high voltage can be developed across it. This is true generally of current sources: short = OK, open = bad, opposite of voltage sources. Each transformer must either be connected to the ammeter or shorted, and the switch cams must be of the make-before-break type so that there is no open-circuit moment as the switch position is changed.

 

[Julie.]

The crucial point here being that the secondary of a current transformer should never be left open-circuit, as a high voltage can be developed across it. This is true generally of current sources: short = OK, open = bad, opposite of voltage sources. Each transformer must either be connected to the ammeter or shorted, and the switch cams must be of the make-before-break type so that there is no open-circuit moment as the switch position is changed.

Correct, that's why there are so many poles on the switch, when it looks like you could achieve the same with just 3.

I suppose people could also miss the details in the switch sequence drawing, it clearly shows overlap in the operation.