CNC machine power

[nizak]

I got called to a local business to run power for a new cnc machining center. The requirements were 3P 240V 50A. After running the EMT and pulling conductors I terminated the run with some bus drop cable into the Xfmr. The Co. had 2 existing machines(the same style and power requirements) they had put in a few years back, as I looked at the termination from the Xfmr to the machine I noticed that the EGC was cut in the cable on both machines and never terminated on the equipment ground block in the machine panel. Both machines had a driven ground rod and that was the extent of it. I asked the owner if he had an EC do the prior work, he said that the EC had only done what I did for the new machine and that the set up guy from the Co. who sold them the machine wired from the Xfmr to the machine and set it up to run. I can't believe that the driven ground(which by the way is stated as being optional in the set up manual) would serve as an effective means if there was a fault. I asked to talk to the set up guy when he shows up tomorrow to see what the reason is for the way he wired the last two. Is there something about the controls on this type of equipment that would be affected by an EGC back to the main service panel? Thanks.

 

[Little Bill]

I got called to a local business to run power for a new cnc machining center. The requirements were 3P 240V 50A. After running the EMT and pulling conductors I terminated the run with some bus drop cable into the Xfmr. The Co. had 2 existing machines(the same style and power requirements) they had put in a few years back, as I looked at the termination from the Xfmr to the machine I noticed that the EGC was cut in the cable on both machines and never terminated on the equipment ground block in the machine panel. Both machines had a driven ground rod and that was the extent of it. I asked the owner if he had an EC do the prior work, he said that the EC had only done what I did for the new machine and that the set up guy from the Co. who sold them the machine wired from the Xfmr to the machine and set it up to run. I can't believe that the driven ground(which by the way is stated as being optional in the set up manual) would serve as an effective means if there was a fault. I asked to talk to the set up guy when he shows up tomorrow to see what the reason is for the way he wired the last two. Is there something about the controls on this type of equipment that would be affected by an EGC back to the main service panel? Thanks.

I have set up (helped) machines similar to these and I don't know of any reason an EGC back to the panel would cause any problems. IMO, too many people have the ill conceived notion that a ground rod is a "catch all" in grounding. They have no idea how the grounding/bonding process works.
I will add that barring some requirement on the xfmr grounding, which I haven't taken the time to look up, I don't know of any problem with an EGC back to the panel.

 

[nizak]

The Xfmr's come pre wired (load side) with a 20' 4 conductor cable. The EGC is terminated at the factory in the Xfmr(tied with all other grounds, case, hinged door, etc) along with the 3 hot conductors on the appropriate voltage taps. The way that the two existing machines arte hooked up, seems that any line to case fault would totally energize all metal parts. Relying on the driven ground to open the CB does not seem like the way to go.

 

[cadpoint]

I've been reading up on exactly this situation. I'm going to refernce this Link Here.

This same situation is detailed and described with graphics amost to the tee, it's at the top of the list!

Understanding bonding and grounding!

 

[don_resqcapt19]

It was (is) common for the manufacturers of this type of equipment to require an installation that violates the rules in the NEC. Those instructions often call for no equipment grounding conductor and a local grounding electrode. An installation made to those instructions is not safe and those instructions should never be complied with.

 

[Fulthrotl]

It was (is) common for the manufacturers of this type of equipment to require an installation that violates the rules in the NEC. Those instructions often call for no equipment grounding conductor and a local grounding electrode. An installation made to those instructions is not safe and those instructions should never be complied with.

i've been doing hook ups and some troubleshooting on cnc stuff since first generation fanuc....
some of this stuff comes from and is used in places in the world, where the electrical code is
interpretive freestyle.

the stuff used to be hugely static sensitive, and poor grounding and floating voltages
of a very few volts played havoc with some of it. the manufacturers specified driving
ground rods, crossing their fingers just for luck. i always pulled a EGC, and made
sure that ground loops were, to the best of my abiltiy, not present.

i've seen bridgeport 3 axis clones that the wiring looked like utility services in india.

i've never had any problem with a decent EGC causing problems with equipment.
a separate ground rod driven by the machine however, can cause ground loops
'cause it's far enough from the building ground reference that there is a difference
of ground potential.

i have at the insistence of the tech setting up the equipment, driven a ground rod
next to, and bonded the equipment to it.... i usually use one of those burndy split
bolt studs, lay the wire thru it unbroken and attach it to the equipment case,
down a 1" flex, unbroken thru the ground rod attachement, and back up to land
on the ground lug of the equipment, unbroken. that ground goes unbroken back
to the grounding bus in the panel.

 

[petersonra]

It was (is) common for the manufacturers of this type of equipment to require an installation that violates the rules in the NEC. Those instructions often call for no equipment grounding conductor and a local grounding electrode. An installation made to those instructions is not safe and those instructions should never be complied with.

One might argue that the transformer creates an SDS and thus the GES connection is required.

 

[don_resqcapt19]

One might argue that the transformer creates an SDS and thus the GES connection is required.

They could say that about the secondary of their transformer, but their instructions sometimes prohibit the supply circuit to have an EGC connected to their equipment.

 

[petersonra]

They could say that about the secondary of their transformer, but their instructions sometimes prohibit the supply circuit to have an EGC connected to their equipment.

You would think someone would have had an accident and sued them over that kind of thing by now.