Control Panel Question.

Thread Starter


Joined Feb 9, 2008
Hi there,
Attached is a photo of a step down transformer from within an industrial control panel.

Consists of:
400V Primary taken from two phases.
Secondary is tapped 230VAC-0 and 24VDC-0.

What I would like to know is why did they tie the 0V of both taps to earth?
Should these not be kept floating with respect to earth?
I have a schematic of supply but the connection (green/yellow) earth wire is not present on either of the 0V taps, yet it has been fitted after.

Regards NM



Joined Jun 7, 2009
there's much discussion on grounding AC circuits in the forum, so a search would give you plenty of results. Generally, it provides you a stable reference and safety in regards to circuit integrity.


Joined Jul 23, 2011
Thats standard practice in many applications, I think it has something to do with reducing the effect of short circuits or unwanted voltages/signals being introduced.

Thread Starter


Joined Feb 9, 2008

This transformer has two secondary taps.
The 230VAC powers a Siemens plc, thats all it does.
The 24VDC is regulated but not smoothed and it supplies power to relays/contactors and solenoid controlled hydraulic valves.

Regards NM


Joined Jul 1, 2008
Well, that throws my theory out the window. There doesn't appear to be any sensitive electronics that would appreciate a ground.



Joined Feb 28, 2009
From what I can see in your picture, the 24V Yellow/Green wire does not tie directly to the winding but rather to the Negative output of the bridge rectifier. As stated by GetDeviceInfo, there have been many discussions on AC grounding. Many machine tools manufactured in the 1950's used isolated secondary windings for relay control power. These usually had two indicator lamps connected in series across the secondary with the center tie point of the lamps tied to machine ground. These indicators would both glow at about 1/2 normal intensity. The theory was a short of any of the control wires to machine ground would not immediately shut the machine off but cause one or the other of the indicators to light brightly while the other would go off. That would indicate a problem and allow for a more controlled shutdown reducing damage to the part being machined.

Thread Starter


Joined Feb 9, 2008
From what I can see in your picture, the 24V Yellow/Green wire does not tie directly to the winding but rather to the Negative output of the bridge rectifier.
Yes thats correct, with a supply like this would it make a difference where it was grounded, straight off the secondary or off the negative of the rectifier?

If off the neg of the rectifier would that mean it is grounded more so for noise cancelling purposes rather than safety as its 24V? Although the 24VDC is only driving resistive coils so wouldnt noise have little or no effect on 24V side?.

panic mode

Joined Oct 10, 2011
not sure where you live but all western countries have own electrical code. in Canada it is CEC (Canadian Electrical Code), in the USA it is NEC (national electrical code) etc.
usually code is mandatory (enforced by law).

CEC insists on grounding secondary of transformers but there is exception as mentioned in 10-112 for example (section 10 is all about grounding and bonding). in short CEC permits isolated circuits (non-grounded secondary of transformer) provided that certain conditions are met (primary and secondary are separated by grounded metal shield etc.)

In Europe, they allow non-grounded secondaries if the transformer has double insulation (must be shown on nameplate of the transformer). this is commonly used in small wall-plug-in style transformers and power supplies.

reason for all this is danger of electrocution from something that is "apparently working" which would give you false sense of security when something can be really really wrong.

suppose you have transformer with 400V primary and 24V secondary. suppose there is accidental short between primary and secondary (due insulation damage, or foreign material).

if the secondary is not grounded, in worst case you can have some 400V in secondary circuit that was expected to only provide 24V. worst of all this circuit would appear to function normally even though it has serious and life threatening failure. btw. this principle is actually used to make auto-transformers.

by using proper practice (grounding secondary and using properly sized overcurrent protection), worst that could happen is blown fuse. this not only stops malfunctioning circuit but also acts as a warning that something is wrong.