Instrument Earthing
- Thread starter Mehdi
- Start date
Thankx RB,
1. Is there any special reason not to ground cables at both side?
2. Should I use a separate earth for Intrinsic Safe instrument rather than normal instruments.
3. Should we connect power earth and instrument earth somewhere?
1. Is there any special reason not to ground cables at both side?
2. Should I use a separate earth for Intrinsic Safe instrument rather than normal instruments.
3. Should we connect power earth and instrument earth somewhere?
Leadfoot
Member
Mehdi said:
1. If you ground the shield at both ends, it can and will conduct current. Even though the potential is low, large amounts of current can flow. With current flowing in the shield, you have magnetic fields inside the shield affecting the signal on the wires. By grounding the shield on one end or one place you set up a proper shield to capture EMI and RFI and ground it.
2. YES!!!
3. Your instrument earth is a gathering point for all instrument shields and instrument earth connections. You can take ONE large cable and connect the INSTRUMENT EARTH BUS to earth ground.
Leadfoot
Member
I do not know of any rule or regulation but every intrinsic panel I have seen had a seperate shield ground for signals in the intrinsic panel.
Intrinsic panels are to help prevent sparks in possible gaseous areas. Would you want an non-intrinsic signal wire there?
Intrinsic panels are to help prevent sparks in possible gaseous areas. Would you want an non-intrinsic signal wire there?
Ken Moore
Lifetime Supporting Member
Depending on the type of I.S. barriers used, the I.S. grounding can be critical.
If you are using the zenner type, then the ground resistance must be less than 10 ohms, but I prefer keeping it 5 ohms or less.
If you are using the zenner type, then the ground resistance must be less than 10 ohms, but I prefer keeping it 5 ohms or less.
Paulus
Member
Hello Ken,
I thought that I'd read somewhere that it was =< 1 ohm?
Maybe this is a UK thing?
I thought that I'd read somewhere that it was =< 1 ohm?
Maybe this is a UK thing?
TPS Reports
Member
Generally it is broken down into two categories Ground Bus "PE" (Safety Ground), and Signal Bus "TE" (Signal Ground to insure equal zero voltage level for each system element).
RMA
Member
I don't know what the regulations are, but under normal circumstances you'll be hard put to get an Earth that stays reliably under 1 ohm. 2 - 3 Ohms was usually about the best we could manage, without resorting to exotic methods.
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