Safety ground is not reliable ground reference

I have written quite a bot of material on Ground loop problems and how to get rid of them. The ground loop related problems are often caused by small ground potential differences between different electrical outlets. Typically those ground potential differences are few volts or less.

Not your fault article by Howard Johnson published in EDN magazine describes situations where those ground potential differences can raise to much higher voltages, up to tens of volts.

The green safety wire, or “third wire,” merely connects the metallic chassis of each product to earth at the ac power entrance. Under ideal, no-fault conditions, the green safety wire carries no current. Based on this an inexperienced designer might therefore conclude that the green wires form a single-point-ground reference system that provides a consistent voltage reference between different ac-powered products.

If live wire during some fault situation touches the metallic chassis, a large fault current back to the power source through the ground wire. The fault current trips the circuit breaker, shutting off power and possibly saving the life. The power shutoff normally takes from a small fraction of second up to few seconds depending on the way the circuit is protected. As the huge fault current surges (up to hundreds of amperes) through the green wire, other devices connected to adjacent outlets can experience voltage differences as large as 60V rms in 120V AC system used in USA. The voltages can be even higher on 230V AC system used in Europe (in theory up to 115V AC, in practice normally less than 70V).

This kind of huge voltage differences between equipment can fry the communications interface on interconnected devices because it is normally outside the operating voltage range most normal interconnection systems can sustain (RS-232, RS-422, RS-485). The thin ground connection on the interconnection cable does not much to reduce the surge those devices get. If you want to design reliably working electronics, it would be better if you design gear that can sustain such extraordinary voltages without damage. If your devices can’t sustain those voltages, it would be better if the equipment would be permanently connected to a common outlet or power strip.

Suitable technologies that can easily sustain 60V or more voltage between devices are Ethernet on UTP wiring (transformer isolated), fiber-based optical links, free space optical communications and wireless RF communications. Traditional RS-232 or RS-485 or RS-422 can be made to withstand this voltage if you take case that the equipment on one of the ends of the communications link or both of them have the communications port electrically isolated (usually uses optioisolators) from the equipment case.

16 Comments

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  5. sondownder says:

    “Traditional RS-232 or RS-485 or RS-422 can be made to withstand this voltage if you take case that the equipment on one of the ends of the communications link or both of them have the communications port electrically isolated (usually uses optioisolators) from the equipment case.”
    Where else can I read about it?

    Reply
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  10. Grounding issues and minimizing EMI « Tomi Engdahl’s ePanorama blog says:

    [...] necessary safety functions. Grounding also have other functions in some applications (for example work as signal ground reference) but the safety should not be compromised in any [...]

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  13. Tomi Engdahl says:

    Intersil
    Over-Voltage Protection for RS-485 Bus Node
    http://www.intersil.com/content/dam/Intersil/whitepapers/interface/overvoltage-protection-rs485-bus-nodes.pdf

    Robustness and reliability have made RS-485 the industrial workhorse over the past 40 years. Its large
    differential signal swing of 1.5V minimum and reliable operation over a wide common-mode voltage range of -7V to +12V have catapulted the RS-485’s widespread deployment. Initially used as a communication network in laboratory instrumentation, RS-485 has spread to control networks in industrial and building automation, PLC networks on the factory floor, process control, commercial heating, ventilation and air-conditioning systems, seismic networks, traffic monitoring systems, and alarm indication systems in oil rigs, coal mines and the petro-chemical industry.

    this white paper focuses on: RS-485 transceiver protection against large over-voltages.

    The 24V and 48V DC supplies in industrial and telecom systems are commonly distributed through the same conduits as the data lines of an RS-485 network.

    If a DC supply shares the same connector or screw terminal block with the data lines of a
    n adjacent bus node circuit, miss-wiring faults can occur that connect one or more supply conductors with the transceiver bus terminals.

    Another failure cause is the layout of the conduit. Sharp bends often violate the minimum cable radius
    specified for data and supply cables. Over time, the increased mechanical pressure on the cable will cause a break in the insulation, causing shorts between power and data lines.

    Engineers new to over-voltage protection often assume that adding external transient voltage suppressors (TVS) to a non-fault protected, standard transceiver ensures protection against short-and long-term over-voltages.

    to protect your bus nodes against the wide range of over-voltages, you need fault-protected transceivers, such as Intersil’s ISL3245xE family. These transceivers provide protection against DC over-voltages of up to ±60V and transient over -voltages of up to ±80V.

    Occasionally the question arises: Why not use a non-fault protected, standard transceiver and a few discrete low-cost transistors with sufficient high voltage breakdown for over-voltage protection?
    The answer is simple: A discrete solution adds more cost and development time, and it consumes more space than a fault-protected transceiver.

    Fault-protected transceivers with common-mode ranges wider than specified in the RS-485 standard require double fold-back current limiting within the driver stage.

    current limiting scheme ensures that the output current never exceeds the RS-485 specification, even at the common modeand fault condition voltage range extremes

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