It is common in many applications in the States. We bond the X0 terminal to the case of transformers and then we ground the case to the GEC somehow. Several different ways to do it but X0, GEC, and the frame all have to be connected. This way any fuses will blow if shorted.

 

Just about every 120 control transformer of any size that I've ever installed or seen has one side grounded.

It's a basic standard practice.

 

A transformer secondary is considered a “separately derived system”

The single location bonding of neutral and ground is normally located the transformer and not at the first means of disconnect. See NEC 250.30(A). Locating the bonding in the transformer helps to stabilize the system voltages. So it is the more beneficial location than the first means of disconnect.

 

From the neutral of your transformer secondary [[120V] you should have two wires from the secondary X2 common side, one White to the controls neutral connection, for controls operating current and one green wire to the control panel chassis and transformer case, to conduct fault current when it occurs. The green wire is dedicated to fault currents when they occur, the white wire is to conduct operating current. They are for separate purposes so that operating current is not forced to flow on the chassis and control enclosure . The separate green wire is dedicated to safely and permanently give any short circuit fault-current to metal enclosures a reliable path back to the source, the secondary of your control transformer. It’s purpose is to blow the control fuse immediately

Do you have a fuse installed on the secondary side to protect it from damage when a fault occurs? Is this why your transformer failed, it’s lacking the proper sized overcurrent protection?

Nothing to prevent overcurrents when a short occurs and you loose the control transformer secondary coil, along with your panel control power.

There are rules for for fusing the primary and secondary’s of transformers that are different depending on type, size location, and number of phases.

But yes you need a neutral and ground from a separately derived system

 

From the neutral of your transformer secondary [[120V] you should have two wires from the secondary X2 common side, one White to the controls neutral connection, for controls operating current and one green wire to the control panel chassis and transformer case, to conduct fault current when it occurs. The green wire is dedicated to fault currents when they occur, the white wire is to conduct operating current. They are for separate purposes so that operating current is not forced to flow on the chassis and control enclosure . The separate green wire is dedicated to safely and permanently give any short circuit fault-current to metal enclosures a reliable path back to the source, the secondary of your control transformer. It’s purpose is to blow the control fuse immediately

Do you have a fuse installed on the secondary side to protect it from damage when a fault occurs? Is this why your transformer failed, it’s lacking the proper sized overcurrent protection?

Nothing to prevent overcurrents when a short occurs and you loose the control transformer secondary coil, along with your panel control power.

There are rules for for fusing the primary and secondary’s of transformers that are different depending on type, size location, and number of phases.

But yes you need a neutral and ground from a separately derived system

Transformer Overcurrent Protection is NEC 450.3

 

If your xformer secondary is floating and not grounded, it sure makes troubleshooting problems more complicated, like in an old fashioned control consolet with multiple push buttons. The quick fix is to take a clip lead and temp ground it, but you best do some investigating first, it may be floating for a reason.

 

Let me further elaborate by admitting that transformers were never my strong point and the grounding for them still sorts of eludes me.

I have attached a picture of the wiring the way it is here. From my, likely wrong, understanding, if you have a device coming off the LV and drawing 8A then those 8A will go going directly to case ground and back to the panel through the neutral.

Can someone correct me?

 

Let me further elaborate by admitting that transformers were never my strong point and the grounding for them still sorts of eludes me.

I have attached a picture of the wiring the way it is here. From my, likely wrong, understanding, if you have a device coming off the LV and drawing 8A then those 8A will go going directly to case ground and back to the panel through the neutral.

Can someone correct me?

ETA: below I am talking about an isolation transformer, which creates a separately derived system with no direct electrical connection between the primary side and secondary side. This would NOT apply to an autotransformer (buck-boost transformer).

Secondary juice has absolutely no reason to want to go to back to the main panel supplying the primary directly. It "wants" to return to it's source, which is the secondary terminals. The wire from the neutral to ground doesn't interest that secondary juice at all.

The current from a neutral load on the secondary side will go back to the neutral terminal on the secondary side of the transformer, then by the voodoo of transformers, through the magnetics / fields / etc. to the primary side, and through the primary side conductors to the supply upstream.

Generally good rule, don't think about things wanting to return to ground too much. The juice you're working on doesn't want to return to ground because it doesn't come from the ground, it wants to return to the transformer. Sometimes the earth / ground is a viable path to that source. (Now other things, static buildups, for example lightning, may want to return to the earth / ground.)

 

Let me further elaborate by admitting that transformers were never my strong point and the grounding for them still sorts of eludes me.

I have attached a picture of the wiring the way it is here. From my, likely wrong, understanding, if you have a device coming off the LV and drawing 8A then those 8A will go going directly to case ground and back to the panel through the neutral.

Can someone correct me?

only the primary winding will send neutral or phase current back to the panel
as stated above
the secondary winding is a separately derived system

neutral current from the secondary winding will not / can not flow back to the panel via neutral or ground wires
because it did not originate from there

the circuit/loop will be from the un-grounded side of the secondary winding
through any switches, or loads
to the grounded side of the secondary winding

 

look at this circuit
you have a supply on the primary winding
and a load on the secondary winding

there is not a physical wire path between them for current to flow between the two windings

the current in the secondary induces the current in the primary by way of magnetism

 

Yes I know there is not a physical path between primary and secondary windings unless it's an auto transformer and that's where I get hung up. If the secondary neutral current cannot return through the transformer, because it is a seperate system, then it has to return through somewhere and the path of least resistance is the ground that it is tied on to.

This is what I have and since, as you said, the secondary winding really isn't connected to anything physically then is the only path back not the ground?

 

what do you need for current to flow ?
a full loop correct ?
from the source and back to the source

look at post #10
remove the primary winding from the picture and you are left with a complete loop / circuit for the secondary winding
it came from there it wants to get back there
and nowhere else
it did not originate in the panel
it does not know the panel exists and doesnt care

it started in the secondary winding and will return to it and no where else

 

I think I'm getting it. Next question is what happens to it in the winding? If it returns there then what happens to the current does it just dissipate?

So then the ground that is attached to the secondary neutral is just purely for reference then because if it won't detect fault current then what does it do? The in line fuse protects against short circuit.

 

you are correct that an auto transformer's secondary current will return to the panel / source that it came from
auto as a prefix implies self (auto mobile, self propelling) it transforms within the same circuit (pri and sec)

what you have is an isolation transformer so named because there is no electrical path between the primary and secondary windings

the primary current is induced by the current drawn by the secondary load

 

when secondary current flows in the secondary winding
it simply goes around and around the secondary circuit

current does not dissipate

the ground connection is to avoid accidental shock IF the secondary winding is shorted in the winding itself
as well as stabilize the voltage of the secondary winding
i.e. make it read 0V neu to ground rather than some arbitrary number between 0 and 120


have you CAREFULLY read all of the previous replies given to you?
ALL of them are correct and can help you understand
IF you will study them

 

another way to look at it :

you dont ground your car battery to the dirt
because the current
started in the battery
goes through the circuit
and returns to the battery

it does not know or care that the earth is there
it does not want to go there

if an electrical system in your car develops a short
it will not shock you if you touch the door handle while standing in a mud puddle barefoot

electricity ALWAYS returns to the negative/return side of the source it came from

 

AC current will shock you if you touch something hot while barefoot in the mud
because the entire system from the generating source to the load (your house or plant)
has a connected neutral that is also connected to ground
in the event of a fault (short) you will become a parallel path to ground and neutral of the system

if you UNground the neutral of the entire system
you can no longer get shocked standing in the mud
because ground/earth is no longer a return path to the source

and yes it will work like that
IIRC initially it was like that

 

more about your isolation transformer

it is called a separately derived system because the secondary is electrically separate from the primary

the only connection between the two is magnetically

 

Are most transformers not isolation transformers?

 

Sample drawing for a control power transformer Single voltage primary, L1 &L2, single voltage secondary X1 & X2
X2 is the control secondary neutral that gets grounded. X1 is the power to controls, X2 is the return (neutral) from the controls. and or fault current from the chassis. Refer back to post 5 for reference to a description.

Note the fuse placement in both primary and secondary windings. For protection of both windings.

 

Auto-transformers do not have isolated primary and secondary windings, its one winding with different taps

 

Thank you Glen for really clear work.

 

Think,,,,,,,,,,,,,, welding machine.

 

Got called in to work to change a control transformer that broke. It was a 600 to 120 and the neutral was tied on to case ground which went back to panel ground.

Is this common? Wouldn't the proper way be to run another neutral back to the panel because sending current back on the ground seems wrong.

Can someone elaborate?

It depends on what the transformer is feeding, but yes, it is quite common in the States.
If it is feeding a lot of low voltage circuits downstream, bonding is necessary. End to end grounding can cause all kinds of weird signaling problems.

 

It depends on what the transformer is feeding, but yes, it is quite common in the States.
If it is feeding a lot of low voltage circuits downstream, bonding is necessary. End to end grounding can cause all kinds of weird signaling problems.

End to end grounding? Like having both ends bonded and not just one end like on shielded cable?

 

This is why I like this site. I can read this stuff every day, read the emerald and green books and still feel like I’m learning when I hear the story told a different way.