Isolating transformer

[IvoS]

I have tripp.lite isolating transformer that has 2x 500VA toroidal transformer inside connected as seen on picture.
I want to disconnect the ground from its secondary side to make it true isolated transformer for measurements and test purposes. My question is regarding the 205J capacitor and varistor. Since there will be no ground connection on the secondary side anymore, should I leave the cap and varistor there or remove it?

[madires]

I think you could leave them where they are.

[Skaface]

i think i agree with madires

[ee.jmlp]

I don't know which measures are you going to do, but if it is for measuring purposes, you must take off the varistor and replace it by a TVS.
(I don't like varistors because they die fast).

Install in the primary an X2 rated capacitor in paralel with the input. You can place the varistor in the input in paralel with the X2 capacitor.
As the final protection, install a GDT, between an output and GND. Select a proper GDT.

With that you will have a true isolation without loosing security and the wire insulation will last forever.

[IvoS]

If you mean GDT as gas discharge tube I have no idea how to size this. Would something like this be suitable? Rated 200V ?
http://www.alliedelec.com/images/products/datasheets/bm/BOURNS/70155366.pdf

[geraldjhg]

put them on the input side
disconect ouput neutral from ground
put in a fuse and a series conected incandescent bulb (with a switch to annull)

voila

[ee.jmlp]

If you mean GDT as gas discharge tube I have no idea how to size this. Would something like this be suitable? Rated 200V ?
http://www.alliedelec.com/images/products/datasheets/bm/BOURNS/70155366.pdf

GDT must be rated at a voltage a bit lower than the maximum V the windings insulation will suffer, let's say as a guidance, less than 500V and higher than 300V. The common value is 470V, like this:
http://es.farnell.com/bourns/2045-47-blf/gas-discharge-tube-470v-10ka/dp/2148113
http://es.farnell.com/bourns/2045-35-blf/gas-discharge-tube-350v-10ka/dp/2148111
 
In europe I would get the 470V one, in USA, the 350V one.
In this case a GDT works discharging unwanted ESD to ground.

[madires]

put in a fuse and a series conected incandescent bulb (with a switch to annull)

I'd prefer an automatic circuit breaker (like the ones for mains wiring) to allow some overload and a nice analog ammeter for showing the current.

[madires]

I don't know which measures are you going to do, but if it is for measuring purposes, you must take off the varistor and replace it by a TVS.
(I don't like varistors because they die fast).

A bidirectional TVS at the secondary is also fine. It reacts faster than a MOV and the energy is limited by the transformers. Correctly dimensioned MOVs take more pain (energy) than a TVS! I wouldn't put a TVS as the only surge protection at the AC powerline, but combining a TVS with a MOV is a nice solution.

Install in the primary an X2 rated capacitor in paralel with the input. You can place the varistor in the input in paralel with the X2 capacitor.
As the final protection, install a GDT, between an output and GND. Select a proper GDT.

Which Gnd? Or do you mean earth/PE? A GDT makes sense for surge protection at the AC powerline but not at the secondary winding. But I wouldn't add a GDT anyway to the isolation transformer. Better get a power strip with a built-in surge protection to power all your T&M gear.

A GDT between PE and one of the secondaries would contradict the purpose of an isolation transformer. It would be just triggered by an earth-referenced overvoltage at the secondary winding. Since the secondary is floating it's not earth-referenced. If the GDT is triggered the secondary would become earth-referenced (current flows through the GDT).   

[ee.jmlp]

Which Gnd? Or do you mean earth/PE? A GDT makes sense for surge protection at the AC powerline but not at the secondary winding. But I wouldn't add a GDT anyway to the isolation transformer. Better get a power strip with a built-in surge protection to power all your T&M gear.

A GDT between PE and one of the secondaries would contradict the purpose of an isolation transformer. It would be just triggered by an earth-referenced overvoltage at the secondary winding. Since the secondary is floating it's not earth-referenced. If the GDT is triggered the secondary would become earth-referenced (current flows through the GDT).   

I meant earth, sorry.
  Don't blame about GDT. Secondary will continue floating after an ESD (micro sec or less). GDT have less than 2pF. The ESD current flows through the GDT! That's the purpose! It only conducts the current of the build up charge! Keep in mind You are ALWAYS referenced to earth! So the hand touch or even the air, the enamel or electronics will suffer if you do not care about ESD.

 It's like a controlled spark gap. And will protect electronics on the secondary. But is my opinion. Other solutions would be ok too.

[Zero999]

I agree, use a GDT. It's also very good for testing things such as old CRTs which generate a lot of static.

[ee.jmlp]

Whatever you do, do not put a GDT in the input (between phase and neutral wires) because when it triggers, the GDT becomes a short circuit until the current stops flowing.

Use them only for discharging ESDs and common mode HV noise, and always use one (only one) between an output wire and earth, NOT between outputs. Remember it's like a spark gap, once it triggers becomes a short until energy goes away.

Sorry for my spelling. Good luck

[HalfSpace]

Hi IvoS,
   This seems like a very strange isolating transformer. Having the secondary connected to earth is odd but may be needed to get this device to pass earth leakage testing. I would be careful in removing the earth connection from the secondary and expecting a balanced in respect to earth isolation transformer.

Best Regards
HalfSpace

[madires]

   This seems like a very strange isolating transformer. Having the secondary connected to earth is odd but may be needed to get this device to pass earth leakage testing. I would be careful in removing the earth connection from the secondary and expecting a balanced in respect to earth isolation transformer.

It might be a medical grade isolation transformer for sensitive equipment. There's a nice video on that topic: