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Dingo Dog
21st Nov 2018, 06:45 PM
Hallo everyone

Have a question for the electricial boffins.

Partner has a electric two pin cord Stihl hedger, during operation on the past weekend, managed to cut the extension lead again. The house has a RCD on the main board covering all GPO’S in the house, the shed has a seperate line from the main board to its own fuse box and RCD, cover all GPO’S in the shed.

Extension cord was plugged into a GPO from the shed when partner cut the cord, how come the RCD did not cut power to the severed extension cord. Is it because the unit has only two pins on the Stihl hedger or could there be another problem within the electrical line.

A qualified registered electrician installed the cable, fuse box and RCD’S to the shed.

Regards

DD

RustyArc
21st Nov 2018, 09:04 PM
RCDs trip when the current going out doesn't match the current coming back, i.e. it's found another path, typically to earth. When the hedger's cable was cut, while there might have been a momentary short as the metal blade cut both conductors, the RCD would still have seen the same, momentarily very high, current in balance, as there was no path to earth.

If the cable had an earth wire, and the blade itself was earthed, then it would have tripped the RCD.

jack620
21st Nov 2018, 09:49 PM
When the hedger's cable was cut, while there might have been a momentary short as the metal blade cut both conductors, the RCD would still have seen the same, momentarily very high, current in balance, as there was no path to earth.

Agreed, but in that case it should have tripped the breaker.

malb
22nd Nov 2018, 12:00 AM
OP says the hedger was fitted with a 2 wire cable, and connected via an extension cable to an RCD protected shed circuit. Hedger then cut the extension cord (earthed 3 wire assumed as 2wire extn cords are not legit) but did not trip the RCD.

As reported above, RCD detects an inbalance in the current flowing in the Active and Neutral cables of the system and trips if this imbalance exceeds a threshold, typically 30mA.

Breaker or fuse in the system detects a prolonged excess current flow in the circuit and isolates the circuit until the fault is found and rectified and the breaker is reset or fuse is replaced.

For the hedger to be approved for 2 wire use (no earth), it must be double insulated, ie there must not be a way that an operator can come in contact with the mains power, so the blade assembly must be electrically isolated from the motor components, switch, cables etc, typically by plastic housing with baffles etc, motor electrics at one end and blade drive at the other, and a non conductive gear system connecting blade and motor.

People with a modicum of sense keep their hands etc well clear of the blade when using a hedger, even if they don't keep the lead well away.

The RCD would not trip when the blade hit the cable unless one of two things happened, either the blade needs to cut the insulation on the Earth in the extn cable and one of Active or Neutral, thus bridging Active or Neutral to Earth and creating a current imbalance, or the cable needs to be jammed in the cutter bar and in contact with either Active or Neutral, and the operator touches the cutter bar to attempt to extract the cable, and creates an alternate current path to ground and hence an imbalance that the RCD could detect and act on.

A breaker or fuse might not trip when the extension cord is cut if the cut does not pass through the outer insulation and the insulation of two of the three conductors in the cable. It is quite possible to expose a single conductor in the cable without tripping either the RCD or breaker/fuse if you do not provide an alternate current path to create an overcurrent or earth leakage situation. However, it is only a matter of time before someone/something will contact the exposed conductor and trip one or the other protective device. This is one of the reasons that mains devices used in anyone's workplace need to be regularly inspected, tested and tagged, to ensure that they are safe to use at the time they are tested.

Theberylbloke
22nd Nov 2018, 11:31 PM
Hi Dingo,

So Malb has pretty much nailed it. I would just add that it may be possible (although unlikely) for the hedger blade to cut through the insulation and one conductor of the cable, bend it clear, cut through another cable and bend that clear before severing the final leg.Thus it has to be touching two legs of the cable to operate. There is also a time element involved. A 30ma RCD can see a current to earth and take up to 400 milliseconds to trip as per AS/NZ 3000 specs. The time taken for a circuit breaker to trip is proportional to both the current through it and the time the current flows through the circuit.

Cheers

russ57
24th Nov 2018, 02:48 PM
I didn't realise rcd trip was potentially so slow.

Was talking with my brother who works in a large power station, he was explaining their 6600v breakers trip in 18ms.


Russ

BobL
24th Nov 2018, 04:10 PM
I didn't realise rcd trip was potentially so slow.
Was talking with my brother who works in a large power station, he was explaining their 6600v breakers trip in 18ms.
Russ

The high the V the more reason to trip them quickly.

Karl Robbers
25th Nov 2018, 02:12 AM
The high the V the more reason to trip them quickly.
Yep, we had a sparky where I worked have an accident with 6.6Kv. He lived, but the ferocity of the flash and the damage done in that split second was very educational.
Electrickery is truly a wondrous force.

BobL
25th Nov 2018, 09:24 AM
Yep, we had a sparky where I worked have an accident with 6.6Kv. He lived, but the ferocity of the flash and the damage done in that split second was very educational.
Electrickery is truly a wondrous force.

In 1982 I experienced a 6kV shock between the tip of my ring finger and the palm of the same hand which was touching an earthed Aluminium electronics cabinet. The protection device in the cabinet tripped so I only got a partial shock and because my hand was touching the cabinet mean that most of the current went that way rather than up my arm and through the rest of me. Never the less the shock threw me a couple of metres across the lab. Over the next few hours a strip of outer layers of skin about 5mm wide between my finger tip and palm turned a grey green colour and eventually blistered and fell off leaving what looked like a second degree burn. Those days we didn't report things like this but I should have gone to hospital.

My uncle working as an electrical linesman in the early 1960s got a serious burn across the knuckles of his gloved right hand when the back of his fingers brushed a 66kV line. The flash took the skin and flesh off to the bone and he was off work for 3 months. He was up a long wooden ladder and luckily had a safety harness on otherwise he would almost certainly have been thrown off the ladder. The long wooden ladder also probably added a high enough resistance to minimise the current which would normally fry anyone that experiences this sort of unprotected HV.

It turns out that the back of the hand is a better place to get a shock than the front. Because current causes muscles to contract, contacts with the back of the hand pull the hand away whereas contacts with the front of the hand can lead to the live object being grabbed and held onto.

Back to the OP. A few years ago I accidentally cut through a live 3 core extension cord with a chainsaw. Bit of a flash and the breakers blew but I didn't feel anything. Plastic handles on the CS probably were enough to protect me.

Theberylbloke
25th Nov 2018, 03:36 PM
I didn't realise rcd trip was potentially so slow.

Was talking with my brother who works in a large power station, he was explaining their 6600v breakers trip in 18ms.


Russ

Hi Russ,

Yes and no. The mechanical part of the circuit breaker may trip in that time, after it's received a trip signal.

However, that trip signal comes from a protection relay. Protection relay types and their settings cover a very wide range of aplications. Some relays operate on a defined time basis for example 3 seconds after a fault begins. Some will have a setting to trip "instantly" once the fault current reaches a certian level. Some are set to trip after a time which varies inversely on the value of the fault current. Generally the larger the fault current the faster it will send a trip signal, this is usually called a "time inverse" setting. There are two or three types of time inverse relays which I think are called "standard inverse","very inverse" and "extreme inverse", however it has been a long time since I worked in that area and I could be wrong on the nomenclature.


People who receive significant high voltage shocks often get minimal treatment for the first two to three days in hospital. There is a lot of dead and dying tissue in the body which may or may not overwelm the victim. Not much can be done during this time. Anyone who survives for three days will likely survive the experience.

Cheers

The Beryl Bloke

jhovel
25th Nov 2018, 09:30 PM
Hi Dingo,

A 30ma RCD can see a current to earth and take up to 400 milliseconds to trip as per AS/NZ 3000 specs.

That sounds about right... lol.
I managed to get a shock a few years ago grabbing both active and neutral bare cable ends in my closed hand but not shorted - to pull the cable through a hole. (I switched off the wrong circuit).
It gave me a pretty good zap that I estimated at the time to have lasted about 10 or 20 cycles, before the RCD cut the power.

Cheers, Joe

eskimo
3rd Dec 2018, 10:46 AM
a reminder to those who do not know.....dont get into a false sense of security by thinking an RCD will protect in all cases ..it wont

take for instance, a person who is insulated from earth..ie standing on a rubber mat ...and comes into contact across live active to neutral.... rcd in this scenario is not going to protect you and you could be electrocuted

electricity can bite hard, so be careful

eskimo
3rd Dec 2018, 10:49 AM
TI estimated at the time to have lasted about 10 or 20 cycles, before the RCD cut the power.



gee how could you think about how long you were being zapped for... for me I would be thinking more along the lines of........holy cr*p...am I going to be alive after this:oo: