Isolation transformer

C

Cursitti

Member
Location
Maryland
Occupation
Journeyman Electrician
good evening all,


I looking for some guidance pertaining to two isolation transformers a client of mine needed to have installed for some RF equipment. This equipment takes 230 V single phase line and neutral. The building is equipped with 208 3 phase so I took single phase 208 to my primary and expected to have 230 V line to neutral on the secondary. Line to neutral. I got 230 V but neutral to ground. I have 120. So then I thought I could bond neutral ground with a grounding electrode and my equipment, grounding conductors for the circuits. That worked, but then while taking amp readings I found the grounding electrode had 3.5 amps on it.

I had the customer hook up his RF equipment to the set up. I had first and everything worked fine. My worry is if I bond the neutral the secondary current will then get onto the grounding electro conductor, and if something were to come loose, it could become a potential shock hazard. I am wondering why this is happening. Also, my understanding of an isolation transformer is that if you read hot to ground, you should read 0 V. Also, the same with neutral. Also, my worry is there is filters that the power hits before going into an RF chamber. This filter, specifies line and neutral. I am worried if there is voltage on the neutral the filter could eventually fail. I would really appreciate the help.
 
Cursitti said:
This equipment takes 230 V single phase line and neutral.
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If the equipment requires 230 volt between hot and neutral isn't that what you're providing? When the system has a neutral (or grounded conductor) the voltage will be 230 volts to ground.
 
Ive been second guessing myself about this all day. But I was told by the manufacturer of the RF filters the neutral on their equipment had to be a neutral grounded conductor. But I’ve done many filters and usually the filter doesn’t care what phase or neutral goes where. And the phase voltage to ground was 120 along with the neutral which was also 120. When I ent between the 2 it was 230. That’s why I then grounded the neutral which it worked fine but I was getting a high current on the grounding electrode.
 
Cursitti said:
good evening all,


I looking for some guidance pertaining to two isolation transformers a client of mine needed to have installed for some RF equipment. This equipment takes 230 V single phase line and neutral. The building is equipped with 208 3 phase so I took single phase 208 to my primary and expected to have 230 V line to neutral on the secondary. Line to neutral. I got 230 V but neutral to ground. I have 120.
Click to expand...

Please send a picture of the transformer nameplate. Do you have a 208V to 230V isolation transformer or a 'boost' transformer?

Since you are a new user you might not be able to post images yet. You might need to enlist a moderator.

Cursitti said:
So then I thought I could bond neutral ground with a grounding electrode and my equipment, grounding conductors for the circuits. That worked, but then while taking amp readings I found the grounding electrode had 3.5 amps on it.
Click to expand...

This sounds to me as though you have a 'boost' transformer, which is _not_ a proper isolation transformer, and that you used a separate grounding electrode rather than connecting to the building existing grounding electrode.

Cursitti said:
I had the customer hook up his RF equipment to the set up. I had first and everything worked fine. My worry is if I bond the neutral the secondary current will then get onto the grounding electro conductor, and if something were to come loose, it could become a potential shock hazard. I am wondering why this is happening. Also, my understanding of an isolation transformer is that if you read hot to ground, you should read 0 V. Also, the same with neutral. Also, my worry is there is filters that the power hits before going into an RF chamber. This filter, specifies line and neutral. I am worried if there is voltage on the neutral the filter could eventually fail. I would really appreciate the help.
Click to expand...

Your understanding of the measured voltage on an isolation transformer is correct if you use a low impedance volt meter. With a high impedance meter the tiny leakage from primary to secondary can cause poorly defined voltage readings. The 3.5A on the ground electrode suggests a low impedance connection from primary to secondary, and with 3.5A on the ground electrode you probably already have a shock hazard (my bet: you would measure 120V from the ground electrode you just installed to the building EGC).

Lots of problems here.
 
Yes, I can not post pics yet. But what I did do was treat it like an isolation transformer and left the GEC off the neutral. So now I have 120 to ground on both the neutral and hot. The clients equipment worked just fine but I want to make this right as it’s driving me nuts. The secondary pic of the name plate shows a single coil with hot and neutral connected. It is also called an isolation transformer by the manufacture as they couldn’t provide me with a buck boost for this application.
 
Also I did try and measure voltage between my electrode and other grounded parts I didn’t have any voltage. My worry is if I keep this grounded someone who doesn’t know any better might loosen something and then create a potential shock. This is why I left the neutral and bond unhooked.
 
So the output is 220 volts when applying 208 volts to H1 and H3. Did you ground the secondary neutral? Where is the GEC connected?
 
Yup, that is an isolation transformer nameplate.

You should be able to ground either L or N and to make it 'neutral', and use the other as your 230V 'hot'.

When you measured the 3.5A on the grounding electrode, was the RF equipment on?
 
Yes that is what I did. I bonded the neutral to my electrode. Then I turned my secondary disconnect on and the power went to the filters without the clients equipment on. That’s when I started to read the high current on the electrode. But if it was a true isolation transformer why am I still reading 120 to ground on both conductors? I connected the GEC to a ground bar multiple electrodes go to. I turned off everything but my stuff and it was the culprit of causing the current. I am wondering if I should leave it like it is with no electrode the clients equipment worked fine when he tested it today. I couldn’t do more tests as he left and I stayed trying to troubleshoot the current when I would bond the neutral.
 
I could not do that inside the transformer as there was little room. So I took my grounds and neutrals down to the secondary disconnect where I used a bonding screw to the neutral bar and connected everything there.
 
Cursitti said:
I could not do that inside the transformer as there was little room. So I took my grounds and neutrals down to the secondary disconnect where I used a bonding screw to the neutral bar and connected everything there.
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Assuming the primary is wired correctly for 208 volts if the one side of the secondary is bonded then the only possible voltage between the N and L terminals on the secondary is 220 volts. Same 220 volts for L to ground. Did you snap any photos of the installation?
 
What sort of meter did you use to measure 120V neutral to ground? High or low impedance?

The next thing I'd try is disconnecting the transformer and taking resistance readings between every terminal, specifically looking for continuity between primary and secondary.

The ground bar you connected to...is the 208V neutral bonded to it?
 
No there are multiple ground bars within this facility all go back to building steel. Tomorrow morning I’m going to try and bond it together again and see what happens. I’m just worried about the current. And no I didn’t get any photos but I triple checked myself to make sure I had hooked the primary up right.
 
Something else you can do is put a clamp meter around both primary conductors at the same time. You should measure zero net current (the way a GFCI works).
 
I’m using a fluke meter to measure voltage to ground on the secondary conductors. Technically if I left this as a true isolation transformer their equipment should work almost like a 208 volt or 240 single phase system. But if they truly need a neutral and while testing I’m thinking the return current on the neutral will ne high on the GEC. I’m almost suspecting the main Bar electrode isn’t hooked up in the 30’ ceiling to building steel or it may be loose.

So if I put a clamp on around the primary conductors and if I did read current what would that tell me? I know I shouldn’t be reading anything and they should be basically canceling one another out.

I was getting 220 and 230 volts line to neutral on both of these transformers but when I put a clamp on around the GEC I was getting the current around the wire. And it got worse when I turned the disconnects on the secondary and the voltage went to the filters after the filters there is no load and just receptacles sitting in a RF chamber. Ready for the client to hook their equipment to and test. Like I said yesterday client tested his equipment when I had the system ungrounded and everything worked fine I’m just thinking of longevity if it stayed like this could it mess the equipment up. Either way his equipment is receiveing 230
 
Obviously there shouldn't be any current on the GEC. So you should disconnect the secondary and check for continuity between the L and N to ground. Do the same with the primary. This ensures no winding is shorted internal to the transformer. Then connect your GEC to ground and check for current with the power off. Current could be coming back from somewhere else and flowing through that connection. Then connect power and check voltage to ground with the secondary open. A low-Z meter is best here. It should be approximately zero.
 
I have found that with 2 single phase 20 amp circuits they are using in the chamber that when they are on I am getting about .35 amps on the GEC. This is just a standard 20 amp plug on the inside of the RF chamber that goes through a filter. It gets worse when I turn on both transformers and turn on the secondary disconnects. I am not there today and client will be testing their RF chamber with these 230 volt circuits. Yesterday the clients equipment worked fine with the neutral ungrounded. My thought is their equipment can still be used with the isolation transformer because line to the neutral is 230 volt but neutral to ground is 120. Still weird I am getting voltage between ground and neutral and ground and hot if this is a true isolation transformer.
 
Cursitti said:
I’m using a fluke meter to measure voltage to ground on the secondary conductors.
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That is probably a high impedance meter (but Fluke does make low impedance versions). The 120V may be a 'phantom voltage', caused by very tiny leakage currents. An open question for you to figure out is if this a solid voltage caused by a fault (or misconnection) or a phantom caused by the tiny leakage through perfectly intact insulation.

Cursitti said:
Technically if I left this as a true isolation transformer their equipment should work almost like a 208 volt or 240 single phase system. But if they truly need a neutral and while testing I’m thinking the return current on the neutral will ne high on the GEC. I’m almost suspecting the main Bar electrode isn’t hooked up in the 30’ ceiling to building steel or it may be loose.

So if I put a clamp on around the primary conductors and if I did read current what would that tell me? I know I shouldn’t be reading anything and they should be basically canceling one another out.
Click to expand...
Exactly, the two primary conductors should cancel each other out. If you do read current when clamping both, then that indicates a fault from primary to secondary.

Cursitti said:
I was getting 220 and 230 volts line to neutral on both of these transformers but when I put a clamp on around the GEC I was getting the current around the wire. And it got worse when I turned the disconnects on the secondary and the voltage went to the filters after the filters there is no load and just receptacles sitting in a RF chamber. Ready for the client to hook their equipment to and test. Like I said yesterday client tested his equipment when I had the system ungrounded and everything worked fine I’m just thinking of longevity if it stayed like this could it mess the equipment up. Either way his equipment is receiveing 230
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Do you have any details on the filters? Maybe the filters are _intentionally_ coupling 60Hz to ground as part of reliably filtering out RF.

-Jonathan
 
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