Power Supply Earth for Bench Testing

Question

I work in a typical electronics lab, with resistive earth mats, ESD straps and isolated DC power supplies to test prototype PCBs. I prefer not to use the power supply earth-ground, and to leave the test setup "floating". Mobile phones and laptops do it just fine, so why can't my PCB, and all its connected equipment? However my colleague is uncomfortable having things floating and says they need a reference (connect Earth to the 0V rail).

1) What are the pros and cons of connecting the power supply to Earth?

2) The oscilloscope ground pin has continuity to Earth. Why?

3) Any relevant referece materials available?

Many thanks!

Answer 1

Ungrounded devices float at some potential. This potential may be set by AC line filtering capacitors or just stray capacitance in the mains transformer or safety capacitor that is from the primary to secondary side.

Connecting two floating devices together makes their potential difference equal but there will be a surge of current at the moment of contact to charge/discharge the capacitances. This is fine if connectors have grounds contacting first and they stay connected while data connections mate. But if there is a break in the ground connection due to bad connection, all that potential difference between data pins may damage chips.

This can be witnessed even at home; try taking for example a TV and some source device that are both ungrounded. Touching their metal cases can cause a tingling or stinging sensation (leakage current) and sometimes a small spark can be seen when connecting the equipment cases with wire. This is the reason it reads in the manuals that equipment must be connected to other equipment while devices are unpowered.

A mobile phone is not connected to other equipment than the charger so having a floating device is not a problem or a hazard by itself.

Laptops have grounded power supplies so they are grounded.

1) Having a grounded/earthed supply means your prototype is always at ground/earth potential so it is safe to connect to other grounded/earthed equipment like PC JTAG adapter or oscilloscope. But there is also a downside for having ground-referenced power supply as it could also create a ground loop. Imagine that power supply is driving 2A into load and the ground wire between power supply and load disconnects. If load is connected to other equipment like grounded PC or grounded oscilloscope, the 2A would keep flowing via earth wiring, oscilloscope ground lead or PC USB cable. This is why I keep my lab power supply ground potential floating when I don't need earth ground referenced supply.

2) Oscilloscopes are grounded for many reason, one of them is safety. Imagine accidetally connecting one scope ground lead to mains live voltage. All metal parts and other scope channels grounds and equipment they are connected to would then become live.

Answer 2

Main reasons for connecting electronic devices to earth is safety and fault detection , also mobile devices have internal earth connection too .

Let's imagine one of your electronic equipment is get damaged and live wire touching to metal case . if your device connected to earth which connected to the case , in this situation your device is short circuited so current will flow from case into earth , it will either trip the circuit breakers or differential relays and you will know a fault happened. But if your device not connected to earth circuit won't be close and your device's case work same as live wire but you won't notice it , then one day your hand will be touch into case and if you are close to the neutral wire circuit will be closed on you so you will electrocuted from it .

Also mobile devices have internal earth connection , phone's case connected into ground of the battery so as I said if any fault happen current flow from case into ground of battery and mobile phones have protection circuit so device cut off the power source to prevent any explosion.

I must add , one of the reason for connecting to earth is system build like that . Even you don't connect your devices into earth , main transformer is connected to earth so It still can flow into earth if it find a way but if you install a insulated transformer into lab you don't need earth as long as live and neutral won't connect nothing happen in this system you can safely hold live wire without any protective equipment but otherwise you need to earth

Answer 3

In addition to very important safety considerations, there is still the issue of static charge. Wind, humidity, clouds, carpets, shoes, clothes, friction, etc. can cause objects, including you, to generate a large voltage. Ground yourself to one floating lab bench and you will transfer your charge to that bench. Carry a sensitive device to another floating bench that has picked up a different charge, and you might violate some voltage spec when you put down the component. Similar to the doorknob spark you can get from shuffling shoes on certain flooring. Ground both benches (and kitchen, garage, etc.,) with some bleed resistance to the same earth, and now you can safely move parts across the building with less chances of zapping something.