Garden Pond. to bond or not to bond

[bernardgreen]

Should the water in a plastic lined garden pond have an electrode to provide a path to ground from the water ? Are there any guidelines for this in the regulations.

From a discussion group

When my garden leaf blower was dropped into the pond it continued to run under water until the plug was removed from the socket. I was told the power would have been turned off automatically if that happened

I followed this up. The RCD protecting the socket it was connected to did not operate. It did operate to the test button. After retrieval the motor was found to be saturated so water was definately in contact with the wiring.

Experiments with a neighbour's pond which has a butyl lining found the impedance between water and ground with 50 volts DC applied is over a Meg ohm. The ground electrode was about 2 feet of 15 mm copper pipe as earth rod in damp soil about a yard from the pool edge.

This suggests to me that while the blower wa in the pond there was no signifcant earth leakage as the water appears to be insulated from ground. I can see the water's potential being about mid way between Live and Neutral, about 115 volts above ground. This suggests a person kneeling on wet ground and putting a hand into the water could receive a serious electrical shock while power was still applied to the equipment.

YES I accept that common sense states turn off power and remove plug from socket before attempting to recover the equipment but not everyone has that common sense.

 

[mikhailfaradayski]

bonding water, great idea!

Where would you drive the rod?

 

[holmslaw]

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[holmslaw]

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[DetlefSchmitz]

This suggests a person kneeling on wet ground and putting a hand into the water could receive a serious electrical shock while power was still applied to the equipment.

Yeah, but the RCD would then operate at the same current though the person as it would have done otherwise. I don't see the problem.

 

[bernardgreen]

Yeah, but the RCD would then operate at the same current though the person as it would have done otherwise. I don't see the problem.

The problem is that the RCD operates around 30mA while 10mA through the body can be fatal.

The other problem ( for the discussion group ) was that a device ( the RCD ) said to protect against all accidents with electricity in the garden had not operated when ( according the propganda ) it should have done. and cut the power. I know why it didn't.

 

[electronicsuk]

Perhaps the members on said discussion group should just accept that RCDs aren't a catch-all that will prevent every form of electrical catastrophe from occurring, and the best thing to do is not to drop your electric leaf blower in a pond.

I would, however, be interested to see the results in that situation were a meter used to display the AC volts between one probe in the edge of the pond and another in the ground next to it.

 

[bernardgreen]

I would, however, be interested to see the results in that situation were a meter used to display the AC volts between one probe in the edge of the pond and another in the ground next to it.

Unfortunately activity in the "test pond" ( my neighbour's pond ) is restricted so mains in there is not an option. I do recall results from testing some equipment for total immersion carried out in the 1980's .

The test rig was metal bucket of water standing on an insulating surface. With 230 volt non earthed equipment operating ( as best it could submerged )under water the potential on the bucket varied ( from memory ) around 100 volts to earth / ground when direct on mains and about zero volts to the mid point tap of the 230 / 230 isolation transformer when isolated from the mains . Suggesting that the complex circuit of live parts, water and neutral parts was creating a voltage divider with the water at the midpoint voltage between live and neutral. That was the expected result.

I cannot recall how much current could be drawn the from the water via the bucket when it was 100 volts above ground other than it was enough to operate a relay and possibly power a small indicator lamp.

 

[MrTinker]

Just a thought, but instead of trying to earth the pond water would it not be better, to replace the cable with three core and have the cpc terminated in say a terminal block in the handle, unable to touch any live parts but where the water could make contact with it.

 

[bernardgreen]

Good idea but my understanding of "regulations" about double insulated equipment is that they ban the use of any earthed parts in the equipment.

"Double insulation means this equipment is safe to use" for its intended purpose in the correct environment

 

[ebee]

Reminds me of a mate of mine - I was going on holiday and a customer rang with a problem . I had rewired his house about 12 months earlier.


"When I plug in my pond pump to the external socket you fitted it trips the RCD but when I plug it into one of my other sockets it does not, the external socket must be faulty" (All sockets where RCD protected).

When I got back off holiday I rang my mate.

"I got a shock!" he says then continued to tell me that when he got there the pump was running from an inside socket via an extension lead .

Ohh I thought, I knew what had happend.

The extension lead was two wire (no earth) so the pump was sitting happily making the pondwater in the liner live and he got hold of it whilst kneeling on the surounding earth, then the RCD tripped.


Damn French pump he said and got a replacement - same thing happend again six months later (No not me mate getting another shock , I mean the pump went failty)

 

[Spark123]

The problem is that the RCD operates around 30mA while 10mA through the body can be fatal.

10mA is considered about the maximum let go threshold. The path of the shock through the human body also needs to be considered as to wether or not it will be fatal, i.e. 1mA across the heart for long enough can interrupt its rhythm.
Wether or not a shock is fatal depends on time as well, a shock of 30mA through the whole body for 300mS ought not to be fatal. A shock of 150mA for 40mS should not be fatal, saying that it all depends on the persons tollerance to electric shocks and any underlying medical conditions.