Resistance of cable shield after grounding
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This post was last edited by wyu2378 on 2016-4-7 at 20:24. An 1×2×1.5 intrinsically safe cable was used for the instrument cable; it was connected directly to the terminals in the DCS cabinet, without going through any junction box, over a distance of approximately 150 meters. The shielding layer has been grounded at one end in the control room. The ground resistance was 0.3. The client requested that the resistance of the shielding layer of the entire cable be measured; the actual value obtained (from the meter on site to the busbar inside the cabinet) was around 7. Another device used an intermediate junction box, and the client also measured the resistance of the shielding layer of the entire cable in this case, but only the shielding layer of the multi-core cable was measured (from the junction box to the busbar inside the cabinet), with the measurement result being less than 1. Therefore, it is believed that there is a problem with the shielding of the 1×2×1.5 intrinsically safe cable. I think single-core cables are thinner than multi-core cables, and it is normal for the shielding layer to have a high resistance. I’m not sure if my understanding is correct.
Give it a push; is there anyone experienced who can guide me? I’m trying to learn*. .
I have a question: when measuring the shielding resistance inside a control room, which two resistances are being measured?
Inside the control room is the shielded grounding busbar to the grounding resistance
The ground resistance should be less than 4 ohms; if it is greater than 4 ohms, there is likely a problem, although it’s not ruled out that the measurement method itself is at fault……
The cause has been identified; it is indeed a problem with the cable. It was found that the material used for the shielding layer was not copper wire, but copper-plated iron wire. Everyone should make sure to inspect the cables carefully when purchasing them in the future.
Manufacturers these days are truly hard to keep an eye on; mainly because the cost of engaging in fraud is too low, and of course there are systemic issues at play. Note that I have encountered cases where people took advantage of the price difference by using compensating wires of the compensation type instead of those of the extension type.
Well, the manufacturer is really despicable; expose them!
Hello, OP. Could you please explain in detail how you conducted the testing? It would help us learn as well. Thank you!
Hello, OP. Could you please explain in detail how you conducted the testing? It would help us learn as well. Thank you!