Suburban Machinery Software Division 37777 Harlow Dr., Willoughby, OH 44094 (440) 951-8974 Fax: (440) 951-1412

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GROUND LOOPS, POWER SURGES,

(AND OTHER NASTY STUFF)

A report from the Suburban Machinery repair bench

By Dan Fritz, President, Suburban Machinery, Software Div.

For the last 12 years, the Software Division of Suburban Machinery has been selling and servicing variety of shop floor communications systems. Our most popular system, PC-DNC, is used by hundreds of shops in the US, Canada, Australia, Mexico, and Europe. In those 12 years, we have encountered almost every conceivable electrical problem, and we believe we can save a lot of people a LOT of money by offering a few simple recommendations. If your shop has ever had problems with BLOWN serial ports on your CNC controls, FRIED components on your computers, or COOKED data communications equipment after a thunderstorm, then READ THIS REPORT!

There are a variety of common electrical problems that can occur in a machine shop environment. Some of these are simply annoying, while others can be highly destructive and even dangerous to your machine operators. In virtually all cases, these problems can be easily and inexpensively prevented. For the sake of this article, we will assume that the 3-phase AC wiring and the single phase 220v and 110vac wiring in your shop has sufficient capacity and meets your local wiring codes.

GROUND LOOPS

Probably the most common problems we encounter are due to ground loops. A ground loop can occur when any two devices are grounded in DIFFERENT PLACES. For example, if your CNC machine has a ground stake driven through the shop floor, and your Personal Computer is grounded to the 110vac wall socket, you may experience a ground loop when you connect them together with a serial data cable. The ground loop occurs when the "ground" on one device has a different electrical potential than the "ground" at the other device. When this happens, an electrical current (which simply seeks the path of least resistance to "ground") is conducted through the data cable. If the difference in potential (voltage) is greater than about 30 volts, the IC chips that send and receive these RS-232 serial signals will simply burn out.

A ground loop can also occur when one device is not grounded well enough. For example, if you have several CNC machines connected to a PC-DNC port controller box, and one of them is not properly grounded, damage can occur when some component (say the coolant pump) on the un-grounded machine shorts out. This puts a voltage on the data line, which tries to find its way to another machine’s ground stake.

POWER SURGES

A power surge is less common, but can be equally destructive. When lightning strikes nearby, your power lines may experience a very sort "burst" of high voltage (maybe even thousands of volts!) that can damage power supplies or blow fuses on any AC powered device. Usually a power surge affects both the 110vac and the 3-phase 220/440 volt systems equally, since both are ultimately tied to the power company’s main power lines. Power surges to the 110vac equipment can usually be handled quite well with one of the commercially available "power-strips" that offer surge protection. On most modern CNC machines, the controls are well protected from surges by their internal power supplies. Some servo amplifiers and spindle drive controllers may be vulnerable to surge, however. These units usually have SCRs (Silicon Controlled Rectifiers) or power transistors connected directly to the 3-phase power lines. In a well designed amplifier, these components are protected by "rectifier fuses", which are designed to blow very quickly in the event of a surge. A common (and expensive) mistake is to replace a blown rectifier fuse with a less expensive "slow-blow" fuse. These fuses simply cannot interrupt the circuit fast enough to prevent the transistors and/or SCRs from blowing also.

ELECTRO-MAGNETIC INTERFERENCE (EMI)

Electro-magnetic interference is caused by a wide variety of equipment. The most powerful sources of EMI are devices that cause an electrical arc, such as welders, EDM machines, or power relays that create sparks across their contacts. Other common sources of EMI are devices that are really "electro-magnets" such as transformers, solenoids, fluorescent light ballasts, motors, etc.. When any device creates a magnetic field, and that magnetic field turns on and off (or fluctuates in any way), it can create EMI.

To better understand EMI, take an AM radio into your shop and tune it between stations. The "popping" and "hissing" you hear is created by the many nearby EMI sources. This EMI is picked up by the radio’s antenna, and made audible by the radio’s speaker. Computers (and CNC controls) are very susceptible to EMI because every wire and every circuit-board land acts like a little antenna, which can drive microprocessor controlled equipment NUTS!

The best way to deal with EMI (besides eliminating the source) is to provide a "shield" that will protect the sensitive equipment from the EMI sources. An EMI shield can be a metal box (as in a CNC cabinet) a braided or foil covering (like the shielding on RS232 cables) or anything electrically conductive that will "intercept" the EMI and conduct it to "ground" or divert it around the protected device. If you’ve ever tried to listen to an AM radio inside a steel-framed building, you can understand how shielding works. A strong radio station in a nearby city may be impossible to receive unless you place your radio in front of a window. The window is simply a "gap" in the building’s EMI shielding.

OUR RECOMMENDATIONS:

In our experience, most of the problems you might encounter with ground loops, power surges, and EMI can be prevented. The trick is to understand what causes these problems because the solutions are usually simple and relatively inexpensive. When one of our PC-DNC customers has a problem with the port controller box, we usually repair it here at our Willoughby Ohio facility. The port controller damage we see on a daily basis is usually preventable, and is nearly always caused by ground loops. Here’s what we recommend:

1) Examine your CNC equipment. If you’re not familiar with high-voltage AC wiring, have a electrician or CNC repairman help you. Each machine MUST BE grounded in some way. Either they must have an individual ground rod driven through the floor (recommended) or they must be grounded with a 4th wire that runs with the 3-phase power lines. To prevent ground loops between machines, all machines must be grounded in the SAME WAY.

2) Take an inventory of the kinds of fuses your CNC equipment uses. Be sure you have spares on hand that are the proper rectifier fuses for the servo amplifiers, and spindle drive controllers. Know how to test and replace these fuses should your shop experience a power surge. If you should replace any of these fuses, and they blow a second time - STOP - you may have damaged SCRs or transistors and more extensive troubleshooting may be required.

3) If your CNC controls are ever "locking-up", or experiencing weird malfunctions that seem random and unpredictable, you may be having trouble with EMI. The reason that an individual ground rod is preferred over a 4th-wire ground is because the ground rod is better able to attenuate the high-frequencies that EMI produces. An ideal ground for effective EMI shielding is a copper rod located as close as possible to the CNC cabinet, and firmly connected to the machine’s ground lug with a large diameter stranded wire.

Do not "cheap-out" on the length of the ground rod or on the diameter of the wire. A ground rod driven into dry sandy soil is not very effective, and a small diameter (or solid conductor) wire has much higher impedance (resistance at high frequencies) than a large, stranded wire. For shielding to work effectively, you want a LOW IMPEDANCE path to ground for the CNCs shielding.

4) If you are connecting computers to your CNC controls with data cables, you must avoid ground loops between machines, AND ALSO you must avoid ground loops between your machines and your computer equipment. Most of the time, the damaged port controllers we see are the result of ground loops between the CNCs and the 110vac ground used by the computer and port controllers. This damage can be prevented in two ways: You can ground the computer equipment in the same way as your CNCs, or you can ISOLATE the computer’s ground from the CNCs. Both methods are effective if done properly.

5) Data cable shielding is also vital to good communications. Whenever you run a data cable in a shop floor environment, be aware that every wire acts like an antenna, picking up all kinds of EMI. To avoid EMI problems, we recommend three steps: First, route your cables as far away as possible from strong EMI sources (welders, EDMs, etc.). Second, choose the best quality shielded cable available. An ideal cable would have a foil shield (for high frequencies), a braided shield (for lower frequencies) and would have a low capacitance rating for better high frequency signals over longer distances. Third, and probably most important, be sure that the shielding in the cables are GROUNDED. The best way to do this is to use die-cast metal hoods over the connectors at both ends of the cable. The metal hood should clamp firmly around the folded-back shield, and should be attached to the devices at both ends with 2 screws. If you are using plastic hoods on your connectors, you should solder a "pig-tail" wire to the shield, and connect it to the shield-ground pin on the data cable (pin #1 on a standard 25-pin serial connector).

6) Isolation and surge protection are necessary in those cases where ground loops between the CNCs and your computer equipment are hard to avoid. If your CNCs are wired to a PC in a far-away location, isolation may be the best solution. There are a variety of isolation and surge-protection devices available. Here at Suburban Machinery, we are now offering an isolation/surge protection kit for our PC-DNC systems. An ideal isolation device uses LEDs and photodiodes to transfer the data signals optically, and prevents the signal and ground wires from one device from touching the wires from the other device. For our PC-DNC systems, the port controllers are typically wired to the CNCs with shielded data cables, and the port controller is wired to the PC with a "host" data cable. If the port controller is located on the shop floor, it can be grounded to the machines. This prevents ground loops between the CNCs and the port controller. The host data cable can then be optically isolated from the PC to prevent ground loops through the computer.

7 ) Power surge protection is necessary when the AC line voltage experiences voltage spikes that exceed the line voltage limits of the device’s power supply. Many computers and CNCs already have some form of surge protection built in to their power supply. For these devices, an external power surge protector is unnecessary. For many inexpensive devices that use a simple power supply (such as our PC-DNC port controllers) a power strip with surge protection may be advisable. These surge suppressers typically have a pair of back-to-back zener diodes that "short out" when the voltage exceeds a certain level. This effectively "cuts off" and spikes that may damage the power supply’s diodes or transistors. Such devices are inexpensive and available at almost any computer store or electrical supply house.

In most cases, damage to your computers and CNC controls can be prevented with proper grounding and EMI shielding. In some shops, however, some damage may occur even though the grounding and shielding methods seem adequate. For these shops, or for shops where the isolation is the only answer, we strongly recommend that you purchase one of our PC-DNC surge suppression/isolation kits. These kits are available in a variety of configurations to suit your application. Even if you’re not using our PC-DNC communication system, the devices we use in our kits have been carefully selected to be the most effective, and the most economical devices available.

In conclusion, here are some DOs and DON’Ts to watch out for:

DO: Use ground stakes at each CNC machine when possible

DO: Use low-capacitance, shielded cables for all RS-232 connections

DONT:Leave CNC cabinet doors open for better cooling (EMI gets in)

DO: Use die-cast metal connector covers on all cable ends (or connect pin #1)

DONT:Locate your computer near a welder or an EDM machine

DO: Use optical isolation on data cables run between buildings or power systems

DONT:Replace expensive rectifier fuses with standard or "slow-blow" type fuses