New flex for hedge trimmer.

Joined
11 Sep 2007
Messages
1,490
Reaction score
50
Country
United Kingdom
Mrs Symptoms has her own hedge trimmer ... an electric jobby; my petrol one is too heavy for her to use. However, even after warnings about technique/safety, she has chopped through to cable twice and I've had to make repairs.

It's happened again but instead of connecting the original (and now much shorter!!!) factory fitted 2-core cable (double-insulated machine) should I fit a 3-core (2 + earth) with the earth snipped back within the trimmer but connected at the plug end? The previous cable cuts didn't trip the RCDs within the CU (both sides of the CU have them) but just popped the fuse in the plug.

Clearly, I'd like to introduce RCD tripping for extra Mrs Symptoms safety so will swapping to a 3 core flex as above be the way to go?
 
Sponsored Links
There will be two answers to this question about bringing the earth out of the equipotential zone that is inside the house.

I would use 3 core with a three hole socket at the end with the earth connected. The matching 3 pin plug on the two core lead from the hedge trimmer.

My reason for using 3 core is as you mentioned cutting a cable that has only Live and Neutral does not trip the RCD but relies on the over current of the short circuit to trip the MCB ( or melt the fuse ) to remove the power and make the end of the cable safe. With a very long extension lead ( high impedance ) that short circuit fault current may not be high enough to trip the MCB before the short circuit is removed. That leaves the cut end of the cable Live.
 
In theory if the conductor ends where still live, once the live conductor came grounded to earth the RCD would detect the imbalance and trip. Does not need a CPC to do this!
 
Sponsored Links
My CU is a Memera 2000 loaded with fuse carriers ... so no MCBs and the outside socket is a spur off one of the downstairs ring mains on a 30amp fuse in the CU. So the only thing preventing fried Mrs Symptoms is the 13amp fuse in the hedge trimmer's plug.

As I've got to replace the cable anyway my logic tells me to go with 3 core as per my original post and bernard's confirmation (thanks for that b) so as to trip the RCD in the CU.

PBD ... I follow what you're saying about imbalance and trip but it didn't happen previously and I always thought an earth conductor was needed for RCD function. Obviously I'm not an electrician so I'm happy to be spanked for my lack of knowledge.
 
When ever a cable is accidently cut NEVER assume that protective devices have operated to remove power and make the cable safe.

ALWAYS leave the cut end of cable where it is and go to where is is plugged in and remove the plug from the socket before touching the cut cable.

If necessary make sure no one ( such as children ) can touch the cut end while you go to un-plug the cable.

An RCD will trip if there is a current to ground be it via the CPC or via a person's body to wet earth but ONLY if that current is high enough to operate the RCD ( typically 25 milliamps for a 30 milliamp RCD ) so a shock hazard does still exist on an RCD protected circuit.

The RCD only limits the duration of a severe shock ( > 30 milliamps ). A shock current of less than 15 milliamps can be fatal in some situations.
 
once the live conductor came grounded to earth the RCD would detect the imbalance and trip. Does not need a CPC to do this!
If the cut end cable is not lying on the ground but hanging in mid air then there is no possibly path to ground. Not until someone grabs the cut end of the cable while some part of their body has a path to ground.
 
Thanks Bernard for the safety advice ... thankfully Mrs Symptoms was warned in the early days by me, as I outlined in my original post ("techniques/safety"), to follow the procedure you outlined in the event of snipping the lead. She does follow the correct switching off procedure ... shame she can't remember to avoid cutting the flex, but what can a guy do???

I'll go and get a length of 3 core.

Thanks again.

p.s. you mentioned long lengths of lead (extension leads). What would you suggest should be the maximum length of lead for a hedge trimmer. I'd prefer to fit a new LONGEST lead to the unit so as to avoid Mrs S from needing to use the extension drum (it's a large garden). The unit is 450W 240V ... is voltage drop an issue with such a tool on the end of a long flex?
 
The maximum length is the length required between furthest hedge and power socket.

My choice would be 1.5 mm orange flexible cable. 0.75 mm would carry the current with an acceptable volt drop over 50 metres but a larger cable is easier to see and ( hopefully ) less easy to mistake for leaf or other vegetation for trimming.
 
IIRC from Portable Appliance Testing, max length for an extension lead is
1.5mm - 15 metres
2.5mm - 25 metres (if you can get it in a 13A plug that is)

It might be possible to use a portable socket box with an RCBO set at 10mA / 3A on the end of the extension lead, providing a shorter fault loop path (and easier to disconnect and reset than walking back to the house)
 
The previous cable cuts didn't trip the RCDs within the CU (both sides of the CU have them) but just popped the fuse in the plug.
My CU is a Memera 2000 loaded with fuse carriers ... so no MCBs and the outside socket is a spur off one of the downstairs ring mains on a 30amp fuse in the CU. So the only thing preventing fried Mrs Symptoms is the 13amp fuse in the hedge trimmer's plug.
Do you have a photo of this CU? As you have me baffled!

The 13A fuse went first because that is what it should have done, discrimination of the circuit means a 13A should blow before a 30A fuse, under fault conditions.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top