The causes and effects of inter-system ground noise

June 12th, 2014, Published in Articles: Vector

 

Non-isolated computer equipment connections such as RS-232, parallel and USB can cause inter-system ground noise which AC power protection equipment cannot correct.

The uninterruptible power supply (UPS) system is essential to any business computer system but it cannot protect the computer completely from all power related problems. Energy management company Schneider Electric has identified installation configurations which pose the most risk.

Most computer power protection equipment is designed to protect the load which is attached to them. Ideally, no connection point to the computer should exist, with the exception of the AC power cord. This gives power protection equipment complete control over noise or transient voltage events which could enter the computer from the outside world.

In reality, however, computers are mostly connected to other devices via network wiring, printer data lines and modem telephone lines. AC power protection equipment cannot stop transients from entering the computer via these data connections. These connections may cause AC power problems to damage hardware or data. This inter-system ground noise cannot be corrected with AC-only power protection equipment.

Safety grounding

All computer equipment has three wire grounding type, AC power connectors. Many types of equipment, such as lamps, have only two-wire type plugs as only are required to deliver AC power. The third wire is a safety ground wire connected to all exposed exterior metal parts on the computer equipment. The grounding connection provided at the wall receptacle is connected within the building to copper water pipes and/or structural metal parts of the building. This arrangement protects the user from electrical shock. For reasons of safety, exposed devices such as RS-232 connectors and printer ports are considered exposed metal parts and the voltage at any connector pin must be ground or present a small voltage with respect to ground. This means that, in nearly all computer equipment, the CPU common wire or ground reference is connected to the safety grounding wire (and the enclosure) of the equipment.

This safety grounding arrangement leads to a conflict in systems which are interconnected via data lines. Every interconnected device has two connections which attempt to establish the common reference voltage for data communications: the common wire in the data line connecting the device to other equipment and the device’s safety grounding wire. The latter is also known as a “ground loop”.

It will be shown here how this loop can cause differences between the common reference voltages between interconnected devices. This can damage both hardware and data.

These ground voltage differences or inter-system ground noise between interconnected devices must not be confused with other power problems such as common mode noise, normal mode noise,

EMI, or RFI. These ground voltage differences are referred to as inter-system ground noise.

Inter-system ground or common mode noise?

Fig. 1: An ideal interconnected system.

Fig. 1: An ideal interconnected system.

Widespread confusion exists regarding the difference between inter-system ground noise and common mode noise. The latter is defined as noise existing between the power conductors, hot and neutral, with respect to the ground conductor.

Inter-system ground noise exists between the ground wires supplying interconnected computers. Free-standing computers which are not connected via data lines will not experience inter-system ground noise.

A key reason for this confusion that power protection equipment can reduce common mode noise but cannot correct inter-system ground noise. Some power protection manufacturers lead users to believe that inter-system ground noise is equivalent to common mode noise because their products correct common mode noise. The literature and popular thinking on this subject mistakenly emphasise common mode noise while ignoring inter-system ground noise because sales and technical literature from power protection manufacturers are the leading sources of information on these subjects.

Causes and effects of inter-system noise

A simplified diagram of an ideal interconnected system is shown in Fig. 1.

The grounding wires in the figure from all of the interconnected equipment come from the same source, ensuring that they start out with the same voltage. No current flows in any of the ground wires and they are not subject to any magnetic fields. This ensures that there can be no voltage drop along the ground wires.

This means all points along all of the ground wires are at the same voltage and there is no inter-system ground noise between devices attached at various points along the grounding system. The oscillographs in Fig. 1 show no current noise.

Fig. 1 shows an ideal scenario, but this is often deviated from and data corroption and damage to hardware result.

Fig. 2 shows an interconnected system experiencing inter-system ground noise. Interconnected systems are subject to different ground noise voltages, causing ground noise current in the data line connecting the two systems. In this case, the ground noise results from noise current injected into the ground system of one of the computers by remote other equipment. Inter-system ground noise can result from a variety of different problems.

Ground noise injection

Although the safety ground wire is meant to ground the equipment enclosure, all computer equipment uses this wire for another purpose – to provide a reference point to filter out unwanted emissions of radio frequency interference from the computer equipment.

Common and normal mode electrical noise on the power line is injected into the ground line by the filter in the power supply. Inferior surge suppressors may contribute to this problem.

The effect is that this injected noise current causes a noise voltage between the common references of the interconnected equipment. Data transmissions can be compromised cecause the noise voltages usually become larger as the distance between the equipment increases.

Ground faults

Where interconnected devices are on different electrical circuits within a building and one of these circuits is shared by other equipment with an insulation failure, a very large fault current will be injected into the safety ground wire before the circuit breaker trips. This will cause a momentary ground voltage surge at computer equipment supplied by the circuit breaker, which trips.

The size of this voltage surge may vary between a few volts and over one-half of the rated power voltage (either 120 or 230 V AC), depending on the quality of the grounding system.

The effect is a voltage difference between the common reference points of the interconnected equipment which may exceed the data signal safety voltage rating. Destruction of input/output drivers and CPU motherboards may be destroyed.

Ground currents

Fig. 2: A practical interconnected system

Fig. 2: A practical interconnected system

These are the most common problem. Many problems may cause the ground voltage supplied to the interconnected devices to differ where the interconnected equipment is supplied by different electrical sub-panels (circuit breaker boxes) in the same or different buildings.

Different wiring systems are possible between separate sub-panels, depending on the electrical code. Most of these inter-panel wiring systems do not guarantee that the ground voltages supplied by the sub-panels will be equal. In many cases, the inter-panel ground connections are themselves part of other ground loops (such as building frame members) unrelated to the ground loops caused by the device data connection.

Many scenarios can give rise to dangerous ground voltage differences between sub-panels. These include nearby lightning strikes to ground; lightning-induced currents in ground loops external to the ground loop caused by device data connections; poor building grounding (e.g. sandy soil); fault currents (breaker tripping) in apparently unrelated buildings or circuits, or nearby electrical repair work.

The effect is a voltage difference between the common reference points between devices, which can easily exceed the data cable safety voltage rating. Destruction of data interface drivers and CPU motherboards can result. Another common side effect is heating of the data cabling.

Lightning induced voltage

In this case, the ground loop acts as an antenna. Nearby lightning strikes create enormous electromagnetic pulse noise, which is received by the “antenna”. A large circulating current in the ground loop can be created, with the effect that current may cause the voltage difference between the common reference points on interconnected devices to exceed the safety voltage rating. Damage to the data interface driver cards is common, and propagation of the damage into the CPU motherboard is possible. The area of the ground loop controls the magnitude of the problem.

In rare cases, other types of equipment may create electromagnetic pulses, which are picked up by the ground loop. NMR or MRI imaging machines are an example.

Breaking ground loops by cutting wires

It may be apparent that the source of all of these problems is the ground loop caused by the electrical safety ground. In some cases, disconnection of this ground at one piece of equipment may prevent problems. However, ground wire disconnection should never be performed because it is illegal and presents a severe shock hazard.

It is sometimes suggested that an isolation transformer be used to solve the ground loop problem. This will not work because all safety agencies require that the ground wire of an isolation transformer be passed between the input and output (only the power wiring may be isolated).

Some equipment immune

Some network wiring is completely isolated and does not create ground loops when used to interconnect equipment. In such cases, inter-system ground noise voltages can exist, but will have no effect. Examples of such immune network wiring are Ethernet and fiber-optic cables.

Conclusion

Inter-system ground noise is a little understood type of power problem which can result in mis-operation or even in the destruction of equipment. The widespread use of Ethernet and optic fiber network connections, which are electrically isolated, has decreased the problem over time.

Non-isolated equipment connections such as RS-232, parallel, and USB can, however, cause situations where inter-system ground noise poses a risk to equipment. Such connections should be kept short and the interconnected equipment should be powered from the same branch circuit.

Contact Ntombi Mhangwani, Schneider Electric, Tel 011 254-6400, ntombi.mhangwani@schneider-electric.com

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