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Ground Loops

 
 

Understanding Grount Loops

A long suffering customer support engineer once shouted in frustration "... ground loops should be taught in junior school". The sentiment is right and the problem significant! The Ground Loop is a significant cause of frustration and errors in measuring systems especially but not exclusively in low cost systems.

The cause of Ground Loop errors is simple to understand but often difficult to identify and resolve. The simplest solution is to employ a measuring device with electrically isolated inputs, however these are typically twice the price of non-isolated measuring devices. Isolation is generally the best solution for process critical industrial situations, where reliability is paramount. For less critical applications, the non-isolated solution is more appealing for its cost effectiveness, but unfortunately requires an understanding of the Ground Loop.

What is a Ground Loop?

A Ground Loop (not a particularly intuitive term) arises in a system when points nominated as Grounds are at different potentials (for various reasons) AND there is more than one electrical path connecting these Grounds AND signal lines are connected in such a way that circulating ground currents are able to flow through one or more signal conductors.

The resulting current that flows in the loop can be very large - in extreme cases hundreds of amps - but is more commonly less than 500 mA. The voltage drop along the wire that is part of the measurement circuit is impressed on the signal voltage thus causing the error. Because this current is variable, it cannot be "calibrated out".

Why Ground Anyway?

   There are two main reasons for grounding measurement systems. The first is safety. Should a high voltage accidentally come into contact with the measurement system, the measurement system would become dangerous. A grounded measurement system would cause a fuse to "blow" thus rendering the system safe.

The second reason for grounding to ensure the measuring system is operating within its operating voltage range. If the system were not grounded, the system could be charged to high voltage levels by static electricity or insulation leakage. At some point, the system's insulation would break down, possibly causing measurement errors in the process.

A third reason grounds may be introduced into a system is due to sensor needs. For example, to obtain the best possible thermal contact and response time, a thermocouple may be welded to the object of interest - hence grounding the thermocouple. Another example is the pH electrode - it is in electrical contact with the fluid being measured.

Solutions (or a Single Ground is a Good Ground)

The simplest and most effective method to avoid ground loop problems is to ensure the measurement system is connect to a single ground point. Eliminate the loop. However this is not always possible as sensors must sometimes be grounded, communications links with computers introduce additional grounds or electrical safety demands multiple grounds. But there are solutions:

Use Differential Inputs

The use of differential inputs (as opposed to single ended inputs) is effective in overcoming ground loop errors as there no current passes down the two measurement wires. However, differential inputs are only as good as their common mode rating. If the ground voltage difference exceeds the rating the errors will be introduced.

Shield Grounds

If shielded wires are used, the shielding should be grounded at one end of the cable only, preferably the measurement system end. If grounded at both ends, ground current will flow through the shield and inductively induce noise into the signals wires (which should be twisted to minimize this effect). Also, it is possible that injecting the loop current into the shield connection points may cause errors in the measuring device or the sensor.

Isolation at the sensor

If possible electrically isolate all the wiring associated with the sensor from the area in which the sensor is installed. This may need to include power supplies. The preferred grounding point is at the measuring device.

Sometimes it is not possible to isolate the sensors or its wiring. In these cases it is necessary to isolate at the measuring device by providing a a fully isolated into for that sensor. If there is only one such sensor in the system, it may be possible to use its ground as the system's ground.

Isolation at the measuring device input

Providing isolation at each of the measuring device's inputs is the best but most expensive solution. Typically such isolation will double the equipment cost. Also, isolation has its limitations - speed and accuracy are often sacrificed to contain the costs.

Isolation at the measuring device output

In small systems a cost effective solution is to provide isolation at the measuring devices output interface such as RS232, Ethernet, USB or modem.

Isolation (floating) of the measuring device

An unconventional but occasionally practical method of isolation in a small setup is to isolate the entire measuring device by allowing it to operate off batteries. The method can provide almost any level of isolation required, but if used to isolate very high voltages, extreme care must be taken against forgetful contact.

Ground Loops in Fully Isolated Systems!

Yes, it is possible! Poor sensor wiring can still lead to Ground Loops as the following diagram illustrates two common situations:

Diagram 2 (Grounded Thermocouple)

In this example the sensor has two ground points - the thermocouple tip and the negative lead at the junction box. Any ground current conducted in the probe sheath will introduce errors despite the fact the measurement device is isolated.

The simple rule still applies - a single ground is a good ground. Sometimes you may have no choice where the ground may be placed (as with a grounded thermocouple grounded for good thermal contact and response time), but remove ALL other grounds.

Causes of Ground Potential Differences

Understanding the cause of ground potential differences can be of some help in minimizing their impact on the measuring system and also in negating effects such as corrosion.

  • Electric trains and trams     Electric traction systems often use the tracks as a the return path for the current drawn from overhead power lines.
  • Ground returns in electrical power systems
  • Ground returns in marine power plants
  • Lightning strike
  • Galvanic corrosion prevention systems
  • Inductively induced currents

Prevention is Better than Cure