Earth Ground vs System Ground (DC common)

Thread Starter

bertz

Joined Nov 11, 2013
327
I am a technician whose job it is to service and repair projection laser equipment. These laser projectors are manufactured primarily by two different European companies, for the present time we will call them company A and company K. The input programming to these lasers is done by computer software and transmitted via DMX.

I recently had a moving head laser (company A) come into the shop where the DMX input board was vaporized and the DMX to analog PCB controlling pan and tilt was also shot. I quickly found the root cause of the voltage surge where one of the terminals of the AC input fuse contacted the case.

In the process of repairing this unit, it occurred to me that the earth ground was tied in to the DC common bus. Be aware that this unit has 6 separate DC power supplies.

This seemed strange to me since in my experience we always kept the DC bus isolated from earth ground. Upon inquiry, I was told that all company A lasers are configured this way. Upon further investigation I was told that all company K lasers are configured with the DC common isolated from earth ground.

So I'm throwing this out for general discussion. What are the pros and cons of each configuration?
 

MaxHeadRoom

Joined Jul 18, 2013
19,202
I principally work in the Electrical machine control enclosure business, CNC machine etc, these units have everything from 5vdc circuitry to Hi voltage 3phase and everything in between, the general practice is to set up a central Star point service earth ground termination point, to this point, all the L.V. common's are taken to this common reference point.
There is also a allied practice called Equi-potential bonding where all metallic parts of the machine are bonded to earth ground.
This may not be exactly the same as the equipment you are involved in, but gives some idea of a common practice.
Max.
 
Last edited:

nsaspook

Joined Aug 27, 2009
6,509
Impossible to answer without knowing the exact electrical configuration (like if DC common bus NEEDS to be at a different potential than earth ground) of the electrical system/communications design and installation site considerations for electrical requirements. Unless there is a valid and good electrical reason not to earth ground my preference is to earth ground the power supply common and then use isolation on communications links if needed.
http://www.ti.com/lit/an/slyt298/slyt298.pdf
Supply isolators such as isolated DC/DC converters, and signal isolators such as digital capacitive isolators, avoid the creation of current loops and prevent current flow between remote system grounds with GPDs of up to several thousand volts. Without a reference to ground, the bus transceivers would be operating from a floating supply. Thus, current or voltage surges caused by lightning, ground faults, or other noisy environments would be able to lift the floating bus common to dangerously high levels.
 

MrChips

Joined Oct 2, 2009
19,428
All metal chassis should be bonded to earth ground. The purpose of chassis ground is to eliminate buildup of harmful static potentials. There should be no system current through earth ground.

The problem with grounding supply to earth ground is you might create current loops flowing in the earth ground, which is bad. Also long cable runs act as antennas and can both pickup and radiate interference.

Your best solution is to create a star COMMON point and then bond to earth ground. Then use isolation on all communication links.

(Just confirming what the two gents have said above.)
 
A view of the problem that's virtually the same as everyone else's, but with a few differences.

grounds, commons or earths are somewhat different.

Earth is pretty much a protective ground meaning it should only carry fault currents.

What you generally do is group the commons together and for the sake of arguments, lets call them analog ground, digital ground, high current ground and earth.

We connect stuff to high current ground because there are high currents involved and usually voltage drops don;t matter much.

Digital ground is basically dirty.

Analog ground becomes a clean ground that's used as a reference.

We now connect all of these together at one point and that one point is earth.

Ideally, you want every ground point to go to the same place, but that's impractical, so it's a compromise.

The AC wiring in a typical house is a compromise. Daisy chaining of outlets when devices use ground as a signal reference can get you in trouble. In the work environment, you have orange outlets that have an independent ground, One is the reference and one is for fault currents and a fault won't raise the reference differently for every instrument. They will all raise together.
 

Thread Starter

bertz

Joined Nov 11, 2013
327
All metal chassis should be bonded to earth ground. The purpose of chassis ground is to eliminate buildup of harmful static potentials. There should be no system current through earth ground.

The problem with grounding supply to earth ground is you might create current loops flowing in the earth ground, which is bad. Also long cable runs act as antennas and can both pickup and radiate interference.

Your best solution is to create a star COMMON point and then bond to earth ground. Then use isolation on all communication links.

(Just confirming what the two gents have said above.)
Thanks to all for the feedback. I was just curious as to how others deal with similar situations.

Just to elaborate a bit - Company A does a daisy chain using a green wire from the ground terminal on the AC input to the case and to the chassis of all power supplies. They then terminate into a terminal block and tie in the DC common bus.

Company K uses a star at a common point tied in to the ground terminal on the AC inlet. However they keep the DC ground bus separate.

In any case I am not looking to change or redesign anything. I play the cards I'm dealt. We have over a hundred projection lasers ranging from 5 watts to 50 watts and I'm paid to keep them out on the road. They're not making money sitting in the shop.
 
When you have a good relationship with a company. That relationship can take on lots of different things.
We can help them and they can help us. We helped them to shape the spectrum of the light they provide and in return they get feedback and fix stuff in return.

Here's something really really silly. I found a 10 Amp rectifier on a power supply that was supposed to carry 40 Amps. They never caught the problem. They offered to fix the unit, but we did have to wait for a re-engineer. It required a new transformer.

The same company did another stupid thing. they tried to run 40 Amps through a PC trace when you didn't have to. Just put the connections on the same lug. I had to make those changes to increase reliability.
they eventually made that part internal and did the upgrade for free during scheduled down time (4-6 weeks) for another maintenance issue that we had to pay for on the same piece that we sent back.

I find out that a company's instrument is destroying our electrical devices. The company was skeptical of my findings, but heard me out and was able to duplicate the problem and were quite surprised. They engineered an external fix ASAP which crippled the other functions of the instrument. When they engineered a permanent fix, it was fixed for free.

Unlike Apple. i was nearing the end of my warranty on an Apple iBook. The modem was randomly disconnecting. I found out if you sent an email while connected to the internet using a PPP connection over the modem with the text containing +++ATH$, a disconnect would happen EVERY TIME.
The escape sequence needed to be <pause>+++<pause>ATH$

I could not get past the gatekeeper. 6 months later they had a firmware fix.

I could not get past the gatekeeper. I will not buy another Apple product from then on.
I also decided to use the Apple platform for our entire department. You never know who holds the cards.

My friend's father had the electrical service changed in his house. It turned out the guy didn't do it right and he told the tech that. The tech refused to fix it until he said, i train guys like you. i can do the inspections, I can easily make you loose your license. He showed him his credentials and it got fixed properly and promptly.

You have numbers. I can't buy your products because this is a much poorer design than company Y and we are having more failures than product Y. In order for us to consider you, please make the necessary changes. Due t the nature of our business, we often get asked what products we use and we won't be recommending yours in the future.
 
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