isolation transformer

R

Thread Starter

rajan

Whether we should use center tap isolation transformer (center tap grounded) or ordinary isolation transformer with either side of secondary grounded?

we want to isolate our instrumentation systems from utility ground. please recommend me which system will be effective.
 
B

Bob Peterson

In either case the secondary would not be isolated from utility ground if you grounded either the center tap or either side of the secondary.

Just why do you want to do this in the first place?
 
The problem with isolation transformer is that for safety you need to ground it. By grounding it you've not completely isolating your system. Sure the line is isolated but not the grounds nor neutrals [ground and neutrals are usually connected together]. Noise would still come in via the ground. The only way to completely isolate, is to float your secondary but you'd need to take the extra insulation and safety precautions.

oj
 
J
If you don't require a dual voltage on your secondary, typically the case with a center tap, then use the standard two wire grounded secondary.
 
D
You always want to ground the secondary of an isolation transformer if possible. If left ungrounded, there is nothing to limit how far above the common mode voltage of the winding may rise above ground, causing failure of insulation, or solid state devices.

Proper grounding of the secondary should not introduce much additional noise into the system, because the ground circuit is single ended.

But the original question was, should you ground the center tap or one of the secondary power leads. The center tap is usually the best choice. It minimizes the highest voltage above ground, and any stray currents appear in phase on the secondary, cancelling each other out.
 
M
A major purpose of an isolating transformer is to (drastically) reduce the magnitude of the short-circuit current. Grounding the secondary has no impact on this issue.

A direct connection to the power line can potentially produce a short circuit of 6,000 - 15,000 Amps (even more if close to MV-LV transformer), whereas an isolating transformer can reduce the short circuit current to a much lower value.

Example: a 230/230 VAC 500 VA transformer (approx 2 Amps ) with say 5% losses can reduce the short circuit current (at the secondary side) to approx. 40 Amps (calculation - SC = In / loss factor - i.e. 2A/5% = 40 A).

Meir
 
C

Curt Wuollet

One good method for choosing a grounding method would be to present what the connected equipment would normally see. I would ground one side of a 117 secondary as this is what 117 volt equipment is configured for. The bypasses, surge suppressors (if used) etc, would then work properly. For 220 I would seriously consider grounding a center tap as that is "normal" for 220 stuff here. Low voltage is a toss up, but if it feeds a rectifier I would let the secondary float and ground the low DC side. Not hard at all.

Regards

cww
 
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