Transformer Secondary Grounding

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RAJH

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I am powering a control circuit and several pieces of equipment (AC unit, DC power supply, etc.) via 240VAC, 1-phase stepped down from 415VAC 3-phase via a transformer. The equipment will be operated in Germany. I have one leg of the secondary grounded, which I understand can then be referred to as neutral, and the other leg hot (240VAC).

My question is; when is it appropriate to leave both legs hot (240VAC) and when should one leg be grounded? Is there a rule for this?
 
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RAJH

Member
I'm still confused.

The transformer secondary supplies 240VAC between the two output wires (X1 & X4) with X2 and X3 tied together. If I tie one leg (X4) to ground I still get 240VAC between X1 & X4. Wiring instructions for some pieces of equipment, such as power supplies, show this method of wiring while other pieces of equipment state that they can operate with one hot 240VAC leg or two hot 240VAC legs. I'm not sure when to ground a leg and when to leave them both hot.
 
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hillbilly1

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I'm not sure, but I think they do use a lot of 240 to ground systems in Germany, but here in the states, to keep confusion to a minimum, I ground X2 and X3 to create a 120/240 volt system. High legs on delta systems confuse enough electricians here already, so I try to keep it familar.
 
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jghrist

Senior Member
The standard voltage in Germany is 400/230Y and the Y is normally grounded. I'm surprised that you don't already have 240V available as ?-n from the 415V 3? transformer.

Some equipment manufacturers require one conductor to be grounded and some don't care.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
RAJH said:
The transformer secondary supplies 240VAC between the two output wires (X1 & X4) with X2 and X3 tied together. If I tie one leg (X4) to ground I still get 240VAC between X1 & X4.
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You will whether you ground oine conductor or not. Grounding a conductor "locks" the other conductors' voltages to earth at whatever their respective voltages are to the grounded one.

Wiring instructions for some pieces of equipment, such as power supplies, show this method of wiring while other pieces of equipment state that they can operate with one hot 240VAC leg or two hot 240VAC legs. I'm not sure when to ground a leg and when to leave them both hot.
Click to expand...
As Hillbilly mentioned, you could ground the junction of X2-X3 like the power company does, which will give you "two hots" and the relative safety of a grounded SDS.

In other words, you'd have the necessary 240v and only 120v to ground at the same time.

For your purposes, I'd look for any equipment that requires the supply to be one way or the other, and ground the conductor that suits that piece.
 
R

RAJH

Member
LarryFine said:
You will whether you ground oine conductor or not. Grounding a conductor "locks" the other conductors' voltages to earth at whatever their respective voltages are to the grounded one.

As Hillbilly mentioned, you could ground the junction of X2-X3 like the power company does, which will give you "two hots" and the relative safety of a grounded SDS.

In other words, you'd have the necessary 240v and only 120v to ground at the same time.

For your purposes, I'd look for any equipment that requires the supply to be one way or the other, and ground the conductor that suits that piece.
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My understanding is that 1-phase power in Germany consist of one hot (240VAC) one grounded neutral and the safety ground. Using this information I configured the transformer this way (X2 tied to X3, X1 tied to ground and X4 @ 240VAC). I am using this 240VAC to power control circuits ( relays, etc), DC power supplies and an air conditioner. As far as I can tell every device I have can be powered via one hot leg and one grounded neutral or two ungrounded hot legs. I'm just not sure which method is best or if there is a valid reason to choose one over the other.
 
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jghrist

Senior Member
Germany is covered by IEC standards. I don't know which one covers standard voltages.

One end of a single phase winding would not be considered a neutral according to the NEC definition. That doesn't mean you can't ground it, just that you shouldn't call it a neutral. Call it a grounded conductor.
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100827-0757 EST

RAJH:

If you are describing a machine control system, then you need to find out what standards must be followed in Germany.

In years past in the US in machine control circuits it was normal to work with an ungrounded secondary for the 120 V control circuitry. Then two 120 V pilot lights were connected in series across this secondary. The mid-point of these lights was connected to chassis (ground). The lights would both glow dim and of equal brightness if there were no shorts to chassis (ground). A short to the metal frame somewhere would cause one light to be brighter and thus indicate a problem.

Gradually over the last 30 to 40 years, with the advent of PLC control, these systems have changed to having one side of the 120 V secondary connected to the chassis, and in turn probably connected to earth. Also there has been some migration from 120 V AC to limit switches, etc., to 24 V DC for these inputs.

.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
RAJH said:
I'm just not sure which method is best or if there is a valid reason to choose one over the other.
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Rules and regs aside, the equipment needs would dictate which scheme is better. If there are none, just pick one.
 
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IslandSparky

Member
If equipment isolates one leg only, you need to ground the non-isolated leg. This is often seen with internal online fusing on the isolated leg only.

In contrast, if you use fuses on both legs, it is unsafe to ground one leg.

If the equipment is designed as range-voltage (200-240 or 120-240), chances are it doesn't matter. Equipment like pumps are much more likely to care-- do you have a single pole starter or two pole?
 
R

RAJH

Member
gar said:
100827-0757 EST

RAJH:

If you are describing a machine control system, then you need to find out what standards must be followed in Germany.

In years past in the US in machine control circuits it was normal to work with an ungrounded secondary for the 120 V control circuitry. Then two 120 V pilot lights were connected in series across this secondary. The mid-point of these lights was connected to chassis (ground). The lights would both glow dim and of equal brightness if there were no shorts to chassis (ground). A short to the metal frame somewhere would cause one light to be brighter and thus indicate a problem.

Gradually over the last 30 to 40 years, with the advent of PLC control, these systems have changed to having one side of the 120 V secondary connected to the chassis, and in turn probably connected to earth. Also there has been some migration from 120 V AC to limit switches, etc., to 24 V DC for these inputs.

.
Click to expand...

I am waiting for a reply from my customer and will use my current design (one hot & one neutral) if they don't know. ACME Transformer tech support recommends my current design for Germany.
 
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