Why Use an Earth Rod when a Disc will do?

Sparkingchip:

Around forty years ago I spoke to a guy I had worked with and he said he had just been discharged from hospital after putting a jack hammer chisel through a 33,000 volt cable whilst digging for a gas pipe.


He said he was lying on a trolley in A&E and a sister came up to him and said “Your boss just phoned, he said to ask you if you will be okay to go back and fill the hole in or has he got to get someone else to do it?”

Typical.


From time to time, ground workers were brought into A&E (nowadays emergency medicine ? ) because a trench had collapsed on them. After having been reassured that there was no life-threatening injury, they wanted to get back to work. No work = no pay!


As for employees, those with the slightest bruise or graze expected a week off. ?

I listened to Paul Meenan and and Tom Nagy discussing foundation earthing on the way home tonight and these Condudiscs were being recommended, but they dodged saying how much they cost, they did say that including one in a quote could make you uncompetitive.

So what sort of price are they?

For one disc and two bags of conducrete (which is what is recommended) it comes to £250. 

the safer installation method and savings on labour installing them safely needs consideration.  Comparing labour costs with drilling rods into the ground with no consideration of service strikes is comparing apples and pears. 

you could however hand dig and put a rod in before you filled it back in if you felt the urge. 

There are plenty of situations where you can be very sure there will be no buried cables or pipes, but a city it not one of them,

Mike

Zoomup said:

If it's a consistent Zs reading of say 300 Ohms, it will easily satisfy the requirements of Table 41.5 up to 100mA operating current R.C.D.s,  including the popular 30mA type. Of course I do not know if you radio people need a lower resistance to true earth for other reasons.

Earth electrode resistances above 200 Ω are considered unstable. This is also stated in the notes to Table 41.5.

Kelly Marie Angel said:

Not very impressive 300 ohms is a bit high maybe if its buried deeper it will be better think ide stick to rods if given a choice

BS 7430 tell us that parts of earth electrodes buried at depths of less than 600 mm may not be effective due to frost and frost-heave. Terminations for buried electrodes (of all types) should be at least 250 mm below ground, in a suitable pit.

There are other alternatives, such as earthing mats (as small as 600 mm square).

In many locations, there are very good reasons why driven rod type electrodes, whilst cheap, may not be the best alternative - not least, risk of underground services strike, or rocky ground at the depths below soil surface we are considering.

Grumpy said:

I've to dig a couple of trenches for swa supplies to a garage and workshop. Would you suggest I lay a tape in there whilst I'm at it? Anything special about terminating the tape? (I've never seen it, let alone installed it!)

As JP says, depends on soil conditions. The electrodes need to be at least 600 mm deep because of frost (BS 7430) ... the SWA is recommended to be at least 500 mm (although Sections 705, 708, 740 etc. require at least 600 mm in general, with Section 705 further requires > 1000 mm in cultivated ground and I would recommend keeping to this for the electrodes also).