EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Houseman on June 12, 2017, 10:44:56 pm
-
Hi folks.
I am scratching my head on a problem that seems unsolvable, at least with my knowledge. Hope this forum can give me some useful hints as usual does.
In a big facility where a fully-automatic sorting-line provides for cardboard boxes drag and drop system, there seems to be ghostly presences that randomly interrupts this complex process.
Now serious. Approx a number of 900 modules provides the sorting of all the incoming boxes and with the use of RFID TAG system, these boxes are routed through the whole sorter, ready for final destination.
Randomly and oddly especially during hot season, light barrier get broken (the LED glow gets faint), fiber optic connection between modules collapses, PLC System acts as crazy causing big missorting etc.etc. then the next day all seems normal.
At first glance the finger was pointed to the AC Supply Line. But a quality power line measurement was performed showing no significant issues:
No extra Harmonic content an a 230V were found, Voltage in normal range, no extra spikes.
So a further in-depth analysis was requested.
Under investigation is actually the earthing system of the whole facility.
At first glance no continuous earthing is provided on the whole metallic structure that is composed of the rails that carries the boxes
Each of these electronic sorting modules attached by side of the rail,(enclosed in a PLASTIC enclosure) is powered by 230VAC line by a L+N+Earth cable. On the same module there is also an extra metallic earth ground screw in order to provide a connection point to the metallic framework of the whole structure (cable duct, sorting rails and so on).
With my newbie surprise the internal PCB of each module shows a thick trace that connects the EARTH 230Vac to the internal 24V DC - low voltage circuit attached to the bridge rectifier.
So far so good- searching on the net and also this forum, I have discovered that this is requested usually with PLC systems. Better protection against electrical shock (even if the enclosure is PLASTIC).Reduced EMI, better ESD protecion.If the negative is not grounded and the positive gets grounded by accident, no fuse will blow but all the signals that were floating around zero will now be at -24 which some input cards won't be able to read, i.e,. the signal input will be -23 to -19 instead of 1 to 5. So every good engineer should recommend this way.
However there are there elsewhere discordant opinions: such connections bring some serious noise inside the DC circuit: ground loops are formed so that floating from earth is somehow recommended.
Now the question: how can ground loops be measured, how can I test, with what instruments (oscilloscopes? Spectrum analyzers? that earth injects noise into the DC side?
Are there some books /application notes I can study? Can static electricity captured by the earth cable flow inside the PCB and cause damage to the internal electronic components?
Any hints is really appreciated.
What is your opinion about Earthing Negative DC side?
Thanks in advice.
Steve
-
Now the question: how can ground loops be measured, how can I test, with what instruments (oscilloscopes? Spectrum analyzers? that earth injects noise into the DC side?
The quickest and easiest way is a high bandwidth current clamp/probe, just put it around each cable (group) you suspect. The power supplies (mains or DC) should cancel each other in each cable and you're left with current from single ended signalling and any undesired loops.