Networked computer equipments and ground loops

Power-related problems were the number one cause of computer downtime (according studies condicted by Contingency Planning Research and Stratus Computer 1994). Many of the power-related problems can be traced to the most basic element of the computer network: The wiring and grounding of the host building.

Majority of today's networks are still attached via a copper-based medium. This copper wire can be the path for distruptive and potentially dangerous current and voltage events.

Typical effect of power related problems are from intermittent reboots or lockups to spark-producing power supply damage. Network interface cards, serial ports, parallel ports and modems are are all prime targets for destructive power events.

Govering standards (in USA)

In the opening page of the National Electric Code (NEC) states that the "purpose of the Code is the practical safeguarding of persons and property from the hazards arising from the use of electricity." The Code fusther states that the "Code contains provision considered necessary for safety. Compliance [with] will result in an installation free from hazards but not necessarily efficent, convient or adequate for good service."

The Institute of Electrical and Electronics Engineers (IEEE) has published IEEE Standard 1100-1992: IEEE Recommended Practice for powering and Grounding Sensitive Electronic Equipment. The main purpose of the standard is to present "recommended engineering principles and practices for powering and grounding sensitive electronic equipment.". The standard attempts to provide a consensus about what specific installation paractices should be used to afford electronic equipment maximum availability.

In all areas the IEEE specifications are more stringent than those contained in the NEC.

Shared neutral problems

Non-linear loads do not draw current in phase with the voltage waveform. This causes distortion and harmonics to current waveforms. As the result of this the currents sent back to the source in neutral wire is additive (the currents do not cancel each other like the case in neutral load case). NEC allows multiple circuits to share a common neutral wire which can lead to overloaded neutral wire if all the circuits are loaded fully with non-linear loads. IEEE standard 1100 specifies the need for dedicated circuit for computer equipment and neutral conducting impedance and sizing requirements.

Use of the counduit as ground

According NEC the conduit or piping that electrical wires is run is allowable as "ground." IEEE standard 1100 recommends that "electronics loads be grounded with separate equipment grounding conductor" and this equipment grounding conductor must be "insulated from the conduit ground."

"Isolated Ground" wiring practices

Isolated circuits were supposed to solve the grounding issues faced by today's computer equipment. In reality the installation of those those circuits is seldom done correctly. The NEC gives very little details about installing the isolated grounds where IEEE Emerald Book gives specific procedures for installation and mainteance of isolated ground circuits. The users of computer systems are suffering this discontinuity between IEEE and NEC guidelines.

Finding the power problems

The on-site work required to find the problem usually starts at the computer equipment under question and works backwards through the electrical distribution infrastructure. Branch circuit are examined for total harmonic distortion (THD), conductor resistance, rms voltage and grounding methodology.

Pay attention to mains connections of connected equipment

Every desktop PC has its cover, as well as the ground pins, connected to the center wire of the mains plug ("protective earth"). If you have two PCs connected to different mains plugs that you intend to connect together, it is possible that each of them is connected to a different branch of your home electrical wiring. Although these are connected together at the switchboard, they may have different voltages if a heavy load is connected to one of them (e.g. washer, dryer, etc.) causing a voltage drop. Single pulse can reach the order of tens of volts. This voltage difference then appears at the inputs of the PC, EASILY DESTROYING the parallel or serial port, hard drive controller, sometimes even the whole mainboard. Therefore, it is a good idea to connect only PC's powered from the same mains plug, or at least connected by a single extension cord. The same applies to monitor, printer and other connections.

Recommendations on grounding in computer networks

Nowadays the shielded LAN cable is becoming common. Shielded cable become grounded in every connection point (both ends of the cable). In this kind of system it is essential to keep the the ground potential difference very low in the whole network area.

Every cabling rack in the distribution points must be grounded to the main grounding bar using a separate ground wire. The electrical system ground must be also very good and at the same potential as the central grounding bar. It is essential to keep the active elements (HUBs, LAN switches, routers) at exactly the same potential as the rack cabinet.

The resistance from any grounding point to the central grounding bar must not me more than 1 ohms and the voltage differece must be less than one volt. The resistance between the workstatio mains power gound in the network cable ground must be less than 2 ohms. In order to keep the grounding resistance low the grounding cables must be thick enough.

In undhielded twisted pair cabling the ground potenatial differences are not as critical as in shielded cabling, but it still a good idea to keep the ground diffeences minumum to guarantee reliable network operaration. Especially the grounding of the equipment racks and the active network equipments must be good.

Sources


Tomi Engdahl <Tomi.Engdahl@iki.fi>