Indoor Pt

[peetey]

I have a few questions about Pts. Is the iron core of a Pt electrically common with the baseplate of the Pt? When you check to see that a Pt is good after a fuse has blown, what values are expected as far as continuity and ohmic values on the primary winding and the secondary winding? If I have a indoor type single fuse 2400:120 or 20:1 Pt can I use it in a delta configuration, and how to arrange this without a double fuse setup. If I have this same Pt and it has a burden rating of 750 VA and has a single 1 amp primary fuse, how much burden will it take to blow the 1 amp fuse and how was this calculated? What will the burden be on this if it is only providing 120 v at the secondary and has no load on the secondary? I'm double checking some of my notes, so any comments would be appreciated. Is there any brand of Pt or ct checker for checking the windings?

--peetey

 

[alehman]

That's a lot of questions. You'll need to talk to the manufacturer on some of these.

Is the iron core of a Pt electrically common with the baseplate of the Pt?

Ask the mfg.


When you check to see that a Pt is good after a fuse has blown, what values are expected as far as continuity and ohmic values on the primary winding and the secondary winding?

Make sure it isn't shorted to ground. I would at least megger it. For winding resistances, you'll have to ask the mfg.


If I have a indoor type single fuse 2400:120 or 20:1 Pt can I use it in a delta configuration, and how to arrange this without a double fuse setup.

Yes, if you remove the fuses. Then connect the three PT's in delta and put the fuses in the three leads between the bus and the delta PT's. This would probably violate any NRTL listings of the PT's.


If I have this same Pt and it has a burden rating of 750 VA and has a single 1 amp primary fuse, how much burden will it take to blow the 1 amp fuse and how was this calculated?

The current ratio is 20:1. Fuses are for protecting against shorts on the primary only. You need secondary fuses to protect against overloads.


What will the burden be on this if it is only providing 120 v at the secondary and has no load on the secondary?

Ask the mfg.

 

[scottf]

The only thing I'll add further to alehman's post is about connecting a VT with a single fuse in delta. I doubt it is possible, since if the VT only has one fuse, it is likely a single bushing VT with the H2 terminal connected to ground via the base. In other words, it's likely only designed for wye connections.

 

[buzzp]

As far as ohmic values of secondary and primary windings, these are small and will likely not tell you anything other than proving continuity. As suggested, measure these windings to ground to verify no shorts. We even doble test our PTs.
If the PT has no load connected to it, then there is no burden on it. The burdens are the loads on the PT or CT, including wire length. The burden of the winding, is not part of the total burden you can place on the PT.
I am not sure of the load on this PT (as far as inductive/capacitive loads) but to get a general idea of the burden that will cause the fuse to blow simply use ohms law (also assuming purely resistive load which is impossible but generally not much reactive loads on these for metering or protection (protection using new digital relays), the old electromechanical relays place a much higher burden on PTs and CTs than the new digital types). Reflect your 1A primary to the secondary by multiplying by ratio = 20 A. You will never get 20A (~2400VA transformer would) out of the secondary @ 120V unless your loads on the PT are highly reactive.