Attic Insulation and Electrics

Bruthal

NewHillel wrote: »

I accept that the earth potential could rise, slightly, if the main neutral fails upstream. However, I cannot envisage any circumstances, other than a badly installed or faulty local earthing system, where the earth potential in a domestic installation could be at, or even close, to 230v. That's entirely leaving aside the fact that the RCCB should trip out.

Don't take my word for it. From the FAS training notes: "The function of the earth electrode is to maintain a connection between the general mass of earth and metallic parts of the consumer’s installation. These can then be regarded as being at zero potential. The earth electrode must be continuously effective and capable of carrying earth leakage and earth fault currents, which may arise."

I dont need to take your word for it, or fas training notes. I have seen it in real life. You are assuming from ideal and perfect scenarios, im talking about real life events tested with meters.

In a tt system, an RCD is used for ensuring a live to earth short trips the circuit MCB. Any idea why this RCD is required?

It is the same reason why neutralizing is used at all. Do your notes tell you why?

I have actually connected 10 amp MCBs direct to earth rods, and the MCBs didnt trip. Guess why?

Often, real life principles vary from text book expectations.

NewHillel

robbie7730 wrote: »

I dont need to take your word for it, or fas training notes. I have seen it in real life. You are assuming from ideal and perfect scenarios, im talking about real life events tested with meters.

I have not suggested that such voltages don't happen - I too have seen them in real life. However, they do not arise where there is a properly installed, and fully functioning earth system. (Last line is not correct.) I readily accept that there are many departures from this ideal - poor earth rod location, prolonged dry spell, earth rod deliberately cut short, oxidised connection etc...

(Oh, it used to be a common practice to wet the ground ner the earth rod, to ensure it passed the test. I have no idea whether that still goes on.)

Bruthal

NewHillel wrote: »

Why do you think that most of the impedance will exist between the ground and the metalwork?

You dont really believe an earth rod has 0 ohms ground connection do you?

I remember almost identical debate about this 3 years ago here, and it wasnt fas notes the poster was reading from.

A single earth rod will have quite a few ohms to ground impedence, and this often increases as a current flows down it to the ground. Current only flows to ground to return the the neutral as the neutral is earthed at multiple points.

Why multiple points? Becasue a single point does not make a low impedence connection.

Bruthal

NewHillel wrote: »

I have not suggested that such voltages don't happen - I too have seen them in real life. However, they do not arise where there is a properly installed, and fully functioning earth system. I readily accept that there are many departures from this ideal - poor earth rod location, prolonged dry spell, earth rod deliberately cut short, oxidised connection etc...

Your not right there. I know your not seeing it because you dont seem to have a solid grasp of it.

A fully functioning earth system is highly dependent on an intact neutral in the neutralized system. Do you understand that part?

If the neutral fails, a solitary earth rod is all that there is to keep the open neutral at ground potential. If it did keep it at earth potential, then the house would operate as normal, and the broken neutral would go un-noticed. It will stay at ground potential if an 11 watt cfl is all thats on. But try turning on the shower. The shower absolutely will not work, but if your earth rod kept the open neutral locked at ground potential, then the shower would work perfectly.

Now, again, the above is exactly why all metal work muxt be bonded in a neutralized installation, but you are not grasping the concept of why.

And also for the same reason, a tt system uses a main RCD covering the entire system, a time delay 300ma one i think they use. This is because the earth rod can not be relied upon to trip MCB`s in earth fault scenarios.

Read this, page 9, section 5.1.2 paragraph ii, carefully. It might say page 13 on the reader, but page 9 of the actual pages themselves. But it can be found from the 5.1.2 anyway

NewHillel

robbie7730 wrote: »

Your not right there. I know your not seeing it because you dont seem to have a solid grasp of it...

You're right, I didn't!

That's me in my box.

Thank you, and goodnight!

Bruthal

NewHillel wrote: »

You're right, I didn't!

That's me in my box.

Thank you, and goodnight!

Well i didnt mean it in a bad way, maybe you are seeing it, and im not:)

cabledude

Ah lads the poor misfortune who submitted the original post has just been bombarded with technical mumbo jumbo : impedance/ ohms / grounding. All well meaning of course but I'd imagine he/she is well lost at this stage.

Getting back to the content of the original post :

If you have rubber insulated energy cables in your home I would strongly advise you to have them checked by a competent electrician. There was a very good reason for the cable manufacturers moving to PVC/LSF sheathing plastic's. They are more durable and safer. I'm not sure how rubber compounds would live alongside insulation. Especially if they were installed 50 years ago. The process used to bind the rubber around the cores(red/black wires) is inferior compared to extruded plastic. I've no idea how well this process would hold together after 50 years. This could be dangerous. Rubber over time will deteriorate. I don't mean to alarm you but if it were my home I'd get them replaced, or at the very least, checked out......

2011

NewHillel wrote: »

Why do you think that most of the impedance will exist between the ground and the metalwork?

Most of the impedance is made up of resistance.

The formula for resistance is:

R = ρl / a

Where:
R = resistance
ρ = resistivity
l = lenght
a = cross sectional area

If you look at the resistance of the general mass of earth you will find that "a" is quite large! This makes R quite small.

Bruthal

2011 wrote: »

If you look at the resistance of the general mass of earth you will find that "a" is quite large! This makes R quite small.

Yea thats the part many do quote, but they forget that a single earth rod, or even a few, dont have a large "a" in contact with the ground.

Also, the impedence measured with a tester, even if low, is unlikely to be reflected for long when a decent current flows through the solitary earth rod.

2011

robbie7730 wrote: »

Yea thats the part many do quote, but they forget that a single earth rod, or even a few, dont have a large "a" in contact with the ground.

I didnt forget. In fact it makes my point as this is part of the circuit between the general mass of earth and the metalwork.

Bruthal

2011 wrote: »

I didnt forget. In fact it makes my point as this is part of the circuit between the general mass of earth and the metalwork.

A now, was not suggesting you forgot, i was agreeing with your post, obviously you dont forget, from your previous posts about it in the thread.

But some only mention the large mass of earth to claim the earth rod path is as good as the neutral path. Which is total nonsense, unless an enormous earthing grid is installed.

NewHillel

2011 wrote: »

Most of the impedance is made up of resistance.

The formula for resistance is:

R = ρl / a

Where:
R = resistance
ρ = resistivity
l = lenght
a = cross sectional area

If you look at the resistance of the general mass of earth you will find that "a" is quite large! This makes R quite small.

Ah Cmon, now, I 'got' it, last night. (Honestly )

Thing is, I never gave it a second thought. Proper earthing giving adequate protection was something I just 'knew'. Its not something I work with it day-to-day. This topic really deserves a sticky thread, all on its own. I'm certain that many tradesmen who should know this, and the implications, don't.