Analysis of instrument wire ground faults when troubleshooting instrument issues on-site
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When dealing with instrument issues on site, engineers or instrument technicians often tell us that the instrument wires removed while fixing such problems must be wrapped with tape, in order to prevent them from coming into contact with parts of the instrument enclosure that are connected to ground. This is done to avoid grounding and thus prevent short circuits in the instrument circuit. Could someone please explain to me in detail why this is necessary?
This is necessary, because if a short circuit occurs, it may damage the related components, resulting in significant losses
It’s good to develop such a *habit – wrap it up properly after removing the stitches. However, this is not an absolute rule; with the advancement of technology, some DCS manufacturers’ products now come equipped with protection against loop short-circuit currents. This means that short circuits on site are no problem – they will neither damage the safety barriers nor the circuit cards. :D
What I want to ask is why there is a short circuit with respect to the ground
The grounding of instruments is divided into working grounding and protective grounding; the grounding of the instrument’s signal circuit (usually the 24V negative terminal of the DC power supply) belongs to working grounding. Most factory grounding systems are equipotentially connected; therefore, if the signal wires in use come into contact with conductive parts such as the instrument enclosures, it will cause a short circuit (the signal wire is connected to ground, and ground acts as the negative terminal of the signal wire, so connecting the positive and negative terminals results in a short circuit). It’s fine; you will measure the positive and negative voltages of the signal, as well as the voltage between the signal line and ground, and find that they are not equal – this is quite normal. The reason, simply put, is that theoretical equipotentiality is difficult to achieve 100%.
This is because 2-wire 4-20mA signals from field instruments are used in a large proportion. In a detection and control system, the point with the lowest potential is usually chosen as the common signal point, and this point is generally connected to ground. In a 4-20mA signal system using a 2-wire configuration, the negative terminal of the 24V power supply has the lowest potential, and it serves as the common point for the system signals. If the instrument wires removed while fixing instrument issues on-site are not wrapped with tape and come into contact with parts of the instrument enclosure that are connected to ground, it is as if those wires are connected to a common point, which may cause a short circuit in the instrument circuit.
Does that mean that if the positive terminal of the signal line is in contact with ground, it will cause a short circuit, but if the negative terminal of the signal line is in contact with ground, there’s no problem?
We need to speed up the planning for Singles’ Day; P; P; P
I’ve done something like this before – once, when replacing a transmitter, there was no insulation tape used, and the 24V positive terminal came into contact with ground, which caused the signal isolator to get damaged. Since then, I’ve been much more careful: lol