I have a general purpose non ventillated 480V to 120 V step down transformer. Would this be considered an "isolation transformer"? I believe it would, but I am confused if it is still considered an isolation transformer if I ground reference one of the load legs of the transformer secondary. Is it still isolated if the secondary is ground referenced?
Isolation Transformers
- Thread starter fifty60
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As far as the NEC is concerned it is a separately derived system and you would have to ground one of the legs to to meet 250.30 and yes it is isolated because there is no electrical connection to the primary.
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"Isolated" is as opposed to an autotransformer, it has nothing to do with grounding. So if there are two separate sets of windings, it is an isolation transformer. We can't see your transformer from here.
Autotransformer diagram:
Isolation transformer diagram:
Autotransformer diagram:
Isolation transformer diagram:
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gar
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As Jraef said grounding of the secondary has nothing to do with isolation.
However, isolation transformer may also mean that there is electrostatic shielding between primary and secondary to minimize capacitive coupling between the primary and secondary.
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As Jraef said grounding of the secondary has nothing to do with isolation.
However, isolation transformer may also mean that there is electrostatic shielding between primary and secondary to minimize capacitive coupling between the primary and secondary.
.
hbiss
EC, New York NEC: 2017
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Exactly. I was going to ask the reason why you are asking if this is an isolation transformer.
-Hal
-Hal
Thanks for the responses. The reason I ask is that normally when I am coming in 120V from the outlet, I add an EMC noise filter. I am thinking that if I am deriving the 120V from secondary of a General Purpose transformer, the noise filter is not needed. Any noise on the 120V side is isolated to the secondary coil. But then I also think that this noise is still going to couple across the windings to the primary side, so adding the noise filter would still be a good idea. I've never seen a noise filter used when a transformer is used, only when coming straight from the wall.
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- Electrical Engineer
Depends on the nature of your "noise". That's why they make SHIELDED isolation transformers. The grounded shield will capacitively couple with any high frequency surges (noise) on the primary side windings and take that directly to ground so that it does not couple with the secondary winding and get to the load.
But an EMC filter is typically filtering out the noise that would be on the SECONDARY side, the load device, from transmitting back to the line it is connected to. Having an isolation transformer HELPS, but would not eliminate the EMC in the same way a filter would.
Yes but ITI is quite a bit cheaper for 120 V. Most filters (Islatrol is very popular) are essentially 2 or maybe 3 pole bandpass filters centered at 60 Hz. Some also have varistor or diode surge protection.
A single phase transformer still passes ground faults. Faults on secondary affect primary and vice versa. Ground fault paths are isolated in three phase delta wye transformers. With the 60 degree phase shift they also block even harmonics. No blocking in wye-wye, delta-delta, or single phase.
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A single phase transformer still passes ground faults. Faults on secondary affect primary and vice versa. Ground fault paths are isolated in three phase delta wye transformers. With the 60 degree phase shift they also block even harmonics. No blocking in wye-wye, delta-delta, or single phase.
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