Impedance and Structural Grounding

[kiki_danc]

These are typical metal roof in my country.. it's connected to iron c-purlins metal screwed to metal sheets. Our structural plans never have any grounding details of this. I'm just wondering what would happen if a live wire touch it or the I-beam connected to it (because we hang electrical wires under the roof), would the entire roof get electrified? and if a person on the other end touch it.. would he get electrified too.. or does it dissipate in large area like this? How do you compute the dissipation behavior of such area given a wire size and current?

And if you ground it (by connecting to the grounding system in the building with neutral connection to centertap of the outside transformer with rods stuck to the soils or when proper grounding is obeyed).. would this trip the circuit breaker? or would the large surface area present the tripping? What do you think?

An unrelated question. Supposed you need a 220-110v step down transformer with same impedance as a 5 meters wire of size 8 AWG. How do you compute how how big or small the transformer must be? Assuming the transformer needs to supply 4 Ampere.

Thank you.

 

[Dullard]

1. There is no 'dissipation.' In the event of a connection between a conductor and the metal structure, I would expect either: a CB trip or the structure to be 'electrified.' Which happens depends on the quality of the unintentional grounding of the structure. Whether a person gets shocked when touching it depends on what else he's touching.

2. If you 'ground it' as described, the circuit breakers should trip. Surface area should not have any effect on that. The ground rods do not have anything to do with this - they are about keeping your 'CB tripping ground' at a potential similar to the local environment - this protects against the 'touch it' problem. The quality of the electrical connections between structural elements needs to be reasonably good - in some cases 'bonding cables' may be required to guarantee continuity across the entire structure. The ground conductor should be at least as big as the largest single conductor strung in the rafters.

3. 5m of #8 wire has a resistance of approx 10 milliohms. One of us is confused? You need something like a 600 VA transformer (assuming some power factor). 110 V * 4 A + margin.

 

[kiki_danc]

1. There is no 'dissipation.' In the event of a connection between a conductor and the metal structure, I would expect either: a CB trip or the structure to be 'electrified.' Which happens depends on the quality of the unintentional grounding of the structure. Whether a person gets shocked when touching it depends on what else he's touching.

2. If you 'ground it' as described, the circuit breakers should trip. Surface area should not have any effect on that. The ground rods do not have anything to do with this - they are about keeping your 'CB tripping ground' at a potential similar to the local environment - this protects against the 'touch it' problem. The quality of the electrical connections between structural elements needs to be reasonably good - in some cases 'bonding cables' may be required to guarantee continuity across the entire structure. The ground conductor should be at least as big as the largest single conductor strung in the rafters.

I'm not talking of just surface area... but the entire iron structure which includes the supporting iron purlins:

If one hot wire touch an inch of it, the entire roof purlins would be electrified (like electrical fence)? But then won't there be too great a resistance to cause the current to flow to the entire structure? Hope to get some kind of table or computations for this, whether if grounded.. a hot wire shorting to it can really trip the breaker.

3. 5m of #8 wire has a resistance of approx 10 milliohms. One of us is confused? You need something like a 600 VA transformer (assuming some power factor). 110 V * 4 A + margin.

 

[Dullard]

Yes. If 1 'Hot' wire touches it anywhere, it will all (very probably) be 'electrified.' The resistance of the steel is very small. The resistance of your structural connections is difficult to estimate, but very likely trivial. A calculation is not going to be much help - you'll need to do actual resistance checks of the 'as-built' structure if you want to proceed with confidence. You'll also need to evaluate how likely the structure is to maintain the measured continuity (fastener corrosion, etc.).

There is a reason that wire isn't just 'hung under the rafters' in countries with a safety-oriented electrical code: It can (arguably) be safely done, but requires a lot of engineering attention. It is difficult to 'double-check' the quality of that effort. Using conduit, proper grounding, and proper materials eliminates most of that necessity - more than a single failure is (usually) required to kill someone.

 

[anorlunda]

An unrelated question. Supposed you need a 220-110v step down transformer with same impedance as a 5 meters wire of size 8 AWG. How do you compute how how big or small the transformer must be? Assuming the transformer needs to supply 4 Ampere.

That makes no sense. What are you trying to accomplish?

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

These are typical metal roof in my country.. it's connected to iron c-purlins metal screwed to metal sheets. Our structural plans never have any grounding details of this. I'm just wondering what would happen if a live wire touch it or the I-beam connected to it (because we hang electrical wires under the roof), would the entire roof get electrified?

I'm not familiar with the electrical codes in the Philippines, but here in the US (and maybe Europe too?), industrial AC Mains wiring is required to be run in grounded conduit, so the fault you are asking about cannot happen.

The question as posed is a dangerous discussion under the PF rules, so this thread will stay closed. Even if the code enforcement in your country is lax, I would strongly encourage you to follow the NEC code rules anyway. You might even consider getting your certification as an electrician, since it appears you are very interested in such things. That would give you a better background to address issues like this.

Best of luck.

http://www.buildersbook.com/mm5/graphics/00000001/2017-NEC-handbook_2.jpg