Grounding an Isolation Xfer

So the logic has it, that if you leave an isolation transformer un-grounded, you cannot be shocked..

Well why would you ground anything with that logic?

Maybe I'm just misunderstanding something here. Please help me and explain.


Thank you!

An ungrounded transformator gives you an IT-supply system.

There are con's and pro's about every supply system, including an IT- supply system.

If I was you I would study the different supplysystems.

I see two problems here. For the sake of argument, your ungrounded system has a
little accident where the 'normally hot' conductor gets grounded (conduit crushed,
wire came loose . .) Now what should be high voltage is at ground potential and
what should be neutral (or ground) is high voltage . . . I think you see that problem?

The other problem is if one is troubleshooting around such a system and is trying to
read earth ground to anything in the system one gets squirrelly readings. (I have
a large panel with 480 volts coming in, a transformer, and it is not grounded . . .
Control wires read to ground do not read right.)

Not to mention that your isolation transformer should have a 'hot' and a 'neutral'.
Between those two wires will be a lethal voltage differential . . .

Should I tell you what little I know about isolated ground panels?
Poet.

ivo.maenen said:

An ungrounded transformator gives you an IT-supply system.

There are con's and pro's about every supply system, including an IT- supply system.

If I was you I would study the different supplysystems.

Click to expand...

Thanks! I will.

Timeismoney08 said:

So the logic has it, that if you leave an isolation transformer un-grounded, you cannot be shocked..

Well why would you ground anything with that logic?

Click to expand...

Why do I ground my transformers? For single phase (120/240V) NFPA 70 / NEC requires it.

So why does NFPA require it? According to Article 250.4(A)(1), the purpose of electrical system grounding is, “To limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines that will stabilize the voltage to earth during normal operation.”

However, documents from the time when they switched from having transformers not grounded to grounded, indicate that the switch was to prevent fires. If a transformer got a short from the primary side to the secondary side of the transformer, the secondary would have a very high voltage to ground, as the primary supply is a grounded supply. The high voltage would break down insulation, let current flow, overheat wiring and cause fires. Likewise, equipment that was grounded would also allow excessive current flow through them, damaging the equipment and causing fires.

This apparently was common enough problem that insurance companies wanted the change. The insulation in transformers wasn't the quality we have today, and they would short internally, or tree branches could fall across the wires going to/from transformers, and etc.

By having the transformers grounded, there is good path to ground, then a the fault would has a high current flow to ground, and fuses will blow reducing the likelihood of a fire.

The trade off, is we have less fires, but get a larger risk of shocks. Back in the early 1900 it was determined that this was the lesser problem. Now that we have things like GFCI's and better fusing/breakers, the risk of electrical shock is even lower.

Actually ungrounded systems were quite common in the North East US
In the old knitting mills most were ungrounded 230V 3 Phase
As it was explained to me many years ago the electrical systems were not very reliable back then.
Phase grounding was common back then.
They were monitored in a very simple way, 3 230V incandescent light bulbs one for each phase to ground. When in normal operation the lights will be emulated at half bright but with a grounded leg the grounded leg would be out and the other 2 lamps would be at full bright. This would tell the maintaince personnel that they have a grounded phase and at end of the shift they would fix the problem. It worked that way for years. But keep in mind in the old days the power to the machines was run 3 heavy power wires running down the middle of the plant when you needed to add a new motor you taped into the wires and added whatever fuses and switches you wanted no code to speak of. Every mill hade their own code standard to follow.
I think it is still listed in the current code but it must be monitored with an alarm when a phase grounds.

There is also 4 wire 2 phase 230V power was common in mills about the same time but I haven’t seen any of them for many years. When I hear people talking about 2 phase power I always have to ask if they are referring to 230/120v 1 phase that is common now, some people wrongfully call that 2 phase because they don’t understand there was a 2 phase power system at one time. That system has 4 wires with 4 fuses and a ground if used the ground was not always used.

The current code requires all power sources to have one leg bonded to earth potential to protect ageist excessive voltage and electrical shock. This would include 24V power supplies. I have seen 24V connections measure 240Vac to ground on systems that are ungrounded. You could get a nasty shock or even damage equipment if it were left ungrounded you and also get erratic operations on some equipment with floating ground. Test interments will sometimes use an isolation transformer to power them so the interment common can be connect to another source to measure it.
Grounding may be the most important connection you can make in a modern power system.
Pay close attention to the local codes on grounding.

proof said:

Why do I ground my transformers? For single phase (120/240V) NFPA 70 / NEC requires it.

So why does NFPA require it? According to Article 250.4(A)(1), the purpose of electrical system grounding is, “To limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines that will stabilize the voltage to earth during normal operation.”

However, documents from the time when they switched from having transformers not grounded to grounded, indicate that the switch was to prevent fires. If a transformer got a short from the primary side to the secondary side of the transformer, the secondary would have a very high voltage to ground, as the primary supply is a grounded supply. The high voltage would break down insulation, let current flow, overheat wiring and cause fires. Likewise, equipment that was grounded would also allow excessive current flow through them, damaging the equipment and causing fires.

This apparently was common enough problem that insurance companies wanted the change. The insulation in transformers wasn't the quality we have today, and they would short internally, or tree branches could fall across the wires going to/from transformers, and etc.

By having the transformers grounded, there is good path to ground, then a the fault would has a high current flow to ground, and fuses will blow reducing the likelihood of a fire.

The trade off, is we have less fires, but get a larger risk of shocks. Back in the early 1900 it was determined that this was the lesser problem. Now that we have things like GFCI's and better fusing/breakers, the risk of electrical shock is even lower.

Click to expand...

So it sounds like we trade the ability to save equipment over the ability to not shock people (unless it gets grounded unintentionally)...? Right?

Just trying to understand.


Thanks for all the help!

Timeismoney08 said:

So it sounds like we trade the ability to save equipment over the ability to not shock people (unless it gets grounded unintentionally)...? Right?

Click to expand...

In the early 1900s when the switch was made by the NFPA, the tradeoff was less likelihood of a fires starting in shorted transformers or from overvoltage on wiring in the event of a fault, but more likelihood of a shock.

Now that we have better insulators, the fire issue isn't as great, but we also have better methods of dealing with the potential of shocks, such as GFCIs.

Note, at higher voltages, like 4160 it is legal to not ground 3-phase system, and many industrial plants run systems like this. On these systems the trade-off is that one ground fault may not take the system down, but high voltages can be induces when faults occur that may cause additional damage. Also, the system needs to be monitored for faults before a second fault occurs.

I would not say that you are more likely to get a shock from grounded system, than 'ungrounded' systems. 'Ungrounded' system are still grounded through capacitance from the wires in the power cables. Ungrounded may give a smaller shock, but I'm sure its still extremely dangerous. I've seen 4160/120V transformers with one of the input feed wires having a blown fuse put out 80 to 90V with enough current (power) to open relays. You will not get me to volunteer to touch a live line on a 'ungrounded' 4160V system.

On an grounded system, a single fault to ground will create a large ground fault current. On an 'ungrounded' system, a faults to ground can induce very high phase to ground voltages. Electronic equipment, VFD's especially is very susceptible to failure under these conditions without additional protective devices.

On these systems the trade-off is that one fault may not take the system down, but additional equipment could fail without additional protective equipment due to voltages induced in the system by the faults. In addition, the ungrounded system needs fault monitoring systems.

proof said:

In the early 1900s when the switch was made by the NFPA, the tradeoff was less likelihood of a fires starting in shorted transformers or from overvoltage on wiring in the event of a fault, but more likelihood of a shock.

Now that we have better insulators, the fire issue isn't as great, but we also have better methods of dealing with the potential of shocks, such as GFCIs.

Note, at higher voltages, like 4160 it is legal to not ground 3-phase system, and many industrial plants run systems like this. On these systems the trade-off is that one ground fault may not take the system down, but high voltages can be induces when faults occur that may cause additional damage. Also, the system needs to be monitored for faults before a second fault occurs.

I would not say that you are more likely to get a shock from grounded system, than 'ungrounded' systems. 'Ungrounded' system are still grounded through capacitance from the wires in the power cables. Ungrounded may give a smaller shock, but I'm sure its still extremely dangerous. I've seen 4160/120V transformers with one of the input feed wires having a blown fuse put out 80 to 90V with enough current (power) to open relays. You will not get me to volunteer to touch a live line on a 'ungrounded' 4160V system.

On an grounded system, a single fault to ground will create a large ground fault current. On an 'ungrounded' system, a faults to ground can induce very high phase to ground voltages. Electronic equipment, VFD's especially is very susceptible to failure under these conditions without additional protective devices.

On these systems the trade-off is that one fault may not take the system down, but additional equipment could fail without additional protective equipment due to voltages induced in the system by the faults. In addition, the ungrounded system needs fault monitoring systems.

Click to expand...

I've never even seen 4160.. haha. Most of the plants I've been, I've only worked with 480V or less.

Thank you for the detailed answers. I really appreciate it.

Timeismoney08 said:

... if you leave an isolation transformer un-grounded, you cannot be shocked...

Click to expand...

I don’t know where you heard this, but that premise is grossly incorrect from the get-go. When you make contact with a live conductor in an ungrounded system, you become part of a capacitor between that source and ground. Whether or not you are injured or killed depends on a lot of circumstances in that capacitive circuit. The best you could say is that you MIGHT not be shocked, but there is a huge difference between “cannot” and “might not” that you would be staking your life on.

Would you ever say that you “cannot” be killed by driving the wrong way on a freeway, just because some people have circumstantially survived it?