Is this design normal ?

If you have a mains power supply inside your box and it is not grounded to earth (as you imply) and you are getting electric shocks then I suggest you do not apply mains power again. You may be electrocuted .Sounds like you have a major fault.I strongly advise you to seek help from a qualified technician who will advise you on safe wiring practices.No hobby is worth dying for !

Hi PC,thanks for you concern on my safety,local electrician may not like to tell you what fault in your box but i curious to know before they touch the box.thank again.

Mowhoong,I suggest that you decide what DC voltage your circuit needs and then buy an external power supply ,an AC/DC adaptor ( regulated) which has the lethal mains voltages safely enclosed inside.They are not too expensive and will allow you to experiment with your circuit safely.Good luck !

Hi PC,Thanks for your advice,i will try,but why the leakage current only ac 5 volts instead of 240 volts,my ELCB will not trip when i connected to the ground ? it really funny ? Best wishes

PC wrote:

Hi Mr.Dan.Thanks on your reply. Best Regards

The real voltage must be more than 5V or you wouldn't feel it. There must be some high impedance leakage path from the mains to the metal box. The voltmeter impedance is low relative to the leakage impedance so it loads down the voltage and only reads 5V.

In general a metal box with mains inside should be grounded for safety purposes. If it is not grounded, then there should be very good insulation to make sure that no failure mode exists that can make the box have a high voltage.

The most likely cause of your situation is that there are capacitors on the mains input connected to the metal box to help with electrical noise immunity and that the metal box is supposed to grounded. I would certainly ground the box if you are going to continue to use this equipment. Grounding is unlikely to make it more susceptable to lightning damage whereas not grounding increases the probability of people damage.

Dan

Hello Jasen.The metal box is just a casing,contain as mentioned above,output have few microswitch for detection pupose,a pair of wire connected to test the DC driver motor, just put the metal box on the assemable line,i have no information of the multi meter which i use to measure the ac 5 volts,electric shock happens when our hand is dry... Bye & Thanks Mow Hoong

not without more information. how is the box cobbected to the electronics inside it what external sensors are attached, are they connected electrically to the box.

where is the box mounted and to what...

it seems odd that the maker didn't ground the metal box, especially when it's got such a leaky powersupply.

have you tried reversing the polarity of the AC supply?

what was the impedance of the voltmeter you used to measure the 5V? from ypur description it sounds like more than 5V (unless you touch it with wet hands)

Bye. Jasen

If there is a transformer and/or other components inside the box and/or mounted to it there may 'capacitive' coupling to the metal box. For example there may be sufficient micro microfards of capacaitance between the winding of a transformer to the metal frame of the transformer for there to be slight AC coupling even at 50 or 60 Hertz! Even if there is no actaul wired connections to the box itself. The 'voltage' you will measure may depend on how you measure this very slight current; it may only be of the order of a few microamps? But if the box not grounded it could possibly 'float' at any voltage up to the AC input voltage to the components inside.

No, the design is not "normal" if an ungrounded bare metal box containing mains AC is exposed to the user. That is a safety issue, and takes precedence over worrying about whether the PLC could be damaged by a lightning strike. Lightning strike surges need to be dealt with outside the box. Whether the box is grounded or not, if a surge can get inside, it can do damage.

Ed

Same leakage could measure 5 volts with one meter and 200 volts with another. Without further information about the meter (manufacturer and model number, or analog or digital, or etc) no one can say what you have measured. But then you should have also measured the leakage 'current'.

If you felt a shock, as others properly noted, the leakage voltage is more than 5 volts - open circuit.

Meanwhile, to have this failure, multiple problems must exist. There exists isolation so that this does not occur AND additional 'safety' circuits so that isolation failures still do not cause shocks. You are not fixing the leakage. You are looking for and identifying multiple failures in your box and building's system.

Even worse, you felt a shock and the ELCB did not trip? Another failure that you must > Hi PC,Thanks for your advice,i will try,but why the leakage current