Surge protection for 4-20mA sensors and data logger?

Common mode or longitudinal mode: point is that current on any or both wires will even pass through that datalogger to obtain earth ground. When discussing these currents, resulting voltages can make linoleum, wood, concrete, and wall paint electrically conductive. Even if all inputs have galvanic isolation, still, that only becomes a capacitor to longitudinal surges. Isolated to AC electric is not isolated to surges. Everything becomes conductive - including the best electrical insulator: air.

All appliances (including datalogger) contain internal protection. So that internal protection is not overwhelmed, earth that Semitron. An earthing connection that is as short as possible ('less than 3 meters'). Same electrode that everything else (including AC power to the datalogger) uses. If the datalogger uses two wire AC, then it is not isolated from earth - when discussing transient voltages and currents. What is isolated for human safety is not isolated for transistor safety. Why? Different voltages define same material 'as and as not' conductive.

Again, refer to that application note:

Both the building (ie datalogger) and antenna tower (sensor) have earthing. So that sensor is not harmed, a Semitron or something equivalent (see also Sidactor and see warning below about its operating voltage) earths sensor end of 4/20 ma wires. As long as the sensor and its wires remain at voltages same as earth beneath, then sensor is not damaged. With a protector on sensor end, then surges from the earth or surges via wire (from other sensors) are not destructive.

Telco does same thing. Lightning to any other building in town is equivalent to a lightning rod connected to their $multi-million computer. Telco does what you are doing - earths wire as it leaves that other building and again earths wire as it enters their building. Earths via a 'whole house' type protector - because destructive surges will find earth ground even destructively through isolated (ungrounded) equipment.

Currently Telcordia (the standards organization for all telcos) is upgrading GR-1089 - their standards for surge protection. For example, gigahertz ethernet cable new standard requires an earthed protector from each wire to earth ground AND no protector between twisted pair ethernet wires. A protector between wires is for surges that typically are not destructive which is why new standards do not call for a protector between wires. Standards are quite blunt about what is necessary for communication wires (such as ethernet): a protector from each wire and short (less than 10 feet) to a single point earth ground.

Appreciate the circuit: a constant current source (not a voltage source) creates destructive surges. Your concern is not voltage. Your concern is current. From first year EE: voltage will increase, as necessary, to conduct that current (which is also how pre-1970s cars created 20,000 volts on spark plugs). If attempting to stop a surge, then voltage will increase as necessary to overwhelm that "dam" (galvanic isolation, the isolated datalogger, or spark plugs). Protection can never stop that current because even 4 kilometers of air could not. Protection means shunting (conducting, diverting, clamping) current to earth so that near zero voltage occurs on datalogger or sensors. This shunting so that component or datalogger protection is not overwhelmed.

Even with an isolated datalogger, voltage will increase as necessary to conduct to earth ground. Again, all electronics have internal protection. Earth a protector so that current will not overwhelm that protection. That datalogger sitting on a table remains a potentially destructive path to earth - via table top.

Why did Ben Franklin invent the lightning rod? Because wood (in church steeples) is an electrical conductor (like a tabletop) and because that electricity cannot be stopped using isolation. Best isolation - 4 kilometers of air - did not stop it.

How would I create an earth ground? Datalogger is AC powered? Then the same 3 meter earth ground rod required electric power source must also be connected 'less than 10 feet' to the Semitron earth ground terminal. Ground wire must have no sharp bends, no splices, not inside metallic conduit, separated from all other non-earthing wires, and remain independent of other earthing wires until all meet at the earthing electrode.

At the sensor end, a one meter ground rod would probably be sufficient. But grounding electrode must contact soil below the frost line. If wire is deep enough, then a one wire solid and bare copper wire could be laid in the trench as an earthing electrode. That wire should be solid, copper, and 6 AWG or larger. Water is not considered sufficiently conductive. Earthing electrode must be in earth - either under water or located where wire enters water. If this is water that people swim in, then that earthing should also connect to anything adjacent that is electrical and earthed (ie overhead lightning).

Appreciate a lesson from scuba diving. When lightning struck the surface, then divers floating free were unaffected. But those touching the bottom were connected to the better conductor - and felt lightning currents.

Generally, 4/20 ma sensors operate at voltages less than 30 volts. Your Semitron conducts nothing until voltages exceed 200. Can your sensors withstand 200 volts applied between any one wire and all other wires (or its body) without damage? Best check those specs for longitudinal mode voltages. Would a 60 volt protector be a better choice? Semitron is the right idea. But its operating and sparkover voltage may be excessive. Telephone appliances can withstand 600 volts without damage. Semitron is for telephone appliance protection. Consult sensor voltage specs. That protector voltage must be below internal sensor protection voltage (max voltage between any two points).

Some first dig a hole, drive an earth> First of all, maybe I should stress that the multiple sensors and the

In gratitude for God not electrocuting him for flying kites in thunderstorms?

(-:

-- Bobby G.