In most control panels, no neutral wire is used, i.e, a line voltage (phase to phase) have been given to a transformer wich gives a phase to ground voltage (normally 220 or 110 volts).

I've heared two reasons for this:
1. To reduce the load of neutral earth (loadability)
2. To isolate the feeding power from the main power network

I suppose that the first reason is fairly right but the second is not completey right, because although the energy is transfered via a magnetic media, every short circuit in one side of the transformer will be transfered to the other side.

1. Is there any other reason for that?

2. What is your opinion about using the above mentioned method instead of simply using a phase to neutral connection to supply the PLCs and other single phase consumers?

3. Normally, one of the two wires of the transformer secondary is connected to earth. Do it must be connected to the Protective Earth (PE), or Neutral or Null Earth (N) in separated systems (TNS or 5 wire systems)?

 

Question 3 is the answer to question two. And direct line power depends on your situation. Your "line" power in many cases, _is_ from a transformer with one side grounded. If so, another transformer may or may not make a difference. If not, a transformer is a really good idea and may be required by codes, etc. so the low side can be at ground potential. Many three phase systems don't have a recognised "neutral".

Regards

cww

 

The point with nr 2 is to prevent ground loops and is probably a bigger reason than nr 1.

Whenever you have a closed ground circuit thru a number of
devices/wires/pieces of equipment/anything else you have a less obvious transformer.

The primary winding in this is all other wires around you with AC current, these gives you a varying magnetic field.

The closed ground loop is a short ciruited one turn secondary winding, and this can, depending on your cirumstances, result in *lots* of current.

This is called a ground loop and can be avoided only by breaking it up All grounding should preferably come from one point and split like the branches of a tree, whenever you connect two such branches you have a ground loop. Note that the equpment is most often made by conducting material (steel) and most often grounded at some point, this point should be the same as your grounding point for the control panel in order to not have any ground loops if any of the devices connected to the panel also are grounded in the equipment.

/Johan Bengtsson