# Hedge trimmer fuse



## Hemsby (9 Nov 2017)

Advice please

My new hedge trimmer is rated 750 watt but was supplied fitted with 13 amp fuse.

For many years I have used the general rule for fuse rating calculation as watts divided by volts (although this is only a guide). In this case it would be 3amp approx, the closest fuse would be 5amps [-o< .

Would there be a surge on starting the trimmer requiring the 13amp supplied :?:


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## novocaine (9 Nov 2017)

more that when the motor gets loaded by cutting thick grass (or the kids toys head off) it will ramp up. 

but stick a 5amp in there (you know you do get 3 amp fuses don't you?), if it pops, you know why.


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## Phil Pascoe (9 Nov 2017)

It seems that anything deemed to be too much for a 3a gets a 13a automatically.


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## sunnybob (9 Nov 2017)

I seem to recall about 20 years ago there was a big to do over fuse sizes and companies started selling plugs without them, making the customer responsible for size of fuse. But if course the average person just kept returning the "defective" plugs, so the idea got dropped, just like the one about only 2 metres of flex and a moulded plug that was all the rage 10 years ago.
As a very rough guide I always worked to 4 amps per kw (3 kw kettle - 13 amp fuse) so 3 amp would in theory run a 750 watt motor, but start up, heavy load, and extra long cable would make a 3 amp very borderline. 5 amp should be fine.


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## porker (9 Nov 2017)

On another point, it is more important with something like a hedge trimmer that it is protected by an RCD than the plug fuse. My wife is expert at cutting the cord and as I still have wired fuses I put a plug with built in RCD. It is fused at 13A as it has no idea what is plugged into it.


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## Phil Pascoe (9 Nov 2017)

sunnybob":10l5uj6x said:


> I seem to recall about 20 years ago there was a big to do over fuse sizes and companies started selling plugs without them, making the customer responsible for size of fuse. But if course the average person just kept returning the "defective" plugs, so the idea got dropped, just like the one about only 2 metres of flex and a moulded plug that was all the rage 10 years ago.
> As a very rough guide I always worked to 4 amps per kw (3 kw kettle - 13 amp fuse) so 3 amp would in theory run a 750 watt motor, but start up, heavy load, and extra long cable would make a 3 amp very borderline. 5 amp should be fine.


I remember talking to a sparky friend about 30 years ago about this, back in the days when most appliances were suppied without plugs - he said the consensus at the time was that most people would use oversized fuses if the plugs were supplied without them - of course this ignored the fact that most plugs purchased came with 13a fuses anyway.


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## sunnybob (9 Nov 2017)

It got really silly. I remember a do it yourself shop that had a box of 13 amp plugs, a box of 5 amp plugs, and a box of 3 amp plugs. Then it went to empty plugs with trays of assorted fuses next to them.
But then the customers kept asking "what size do i need for a lamp/ tv/ whatever?" and their "sales assistants" either did not know (mostly) or were too frightened of being sued to say.
I recall the craze lasted less than a year before the manufacturers quietly deep sixed it and went back to 13 amp.

Porker, if you own that house you really should replace those wired fuses. your house could burn down before one of those would blow, and your insurance would not pay you one penny when they saw that box. If youre renting, go to citizens advice because its not allowed any more.

My first hedge trimmer caused great excitement in my 13 year old son. Dad, Dad, cmon dad, lets have a go!

I handed it to him and BEFORE I could say dont cut the cable, whoosh, he went round in a complete circle with it and couldnt understand why it died. He still doesnt "do" electrics, he's a carpenter!


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## Phil Pascoe (9 Nov 2017)

I remember watching my uncle dancing around the wet lawn after he'd wired an extension with a flat connector with the male side live .............. :lol:


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## sunnybob (9 Nov 2017)

ah, they dont make idiots like they used to....


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## RossJarvis (9 Nov 2017)

sunnybob":2jb2cxnq said:


> ah, they dont make idiots like they used to....



No, they're better, with all the silly person-proofing that goes on by manufacturers means that evolution has produced a better silly person!!


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## porker (9 Nov 2017)

sunnybob":1yelztve said:


> Porker, if you own that house you really should replace those wired fuses. your house could burn down before one of those would blow, and your insurance would not pay you one penny when they saw that box. If youre renting, go to citizens advice because its not allowed any more.


The electrical system is being overhauled with an upcoming large renovation and I will be putting in a system compliant with 18th Ed. regs. Just to be clear though there are no rules about using these systems as long as they are maintained and safe and this doesn't affect the insurance simply because it uses an old hardwired fuse system.


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## Hemsby (9 Nov 2017)

Thanks guys for the replies.

I have always had a “thing” about correct fuse sizes and keep a supply of 3-5-7-10 fuses to hand. Needless to say there is a box of salvaged 13amp.

We had the bungalow rewired 5 years ago including a new consumer unit with several trip devices so think we are covered on that side.


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## sunnybob (9 Nov 2017)

My wife inherited a very small flat 3 years ago. Not really worth selling at this stage. we used a letting agent and were told we could NOT rent it out untill the old wired fuse electrical box was replaced with mcb's.

Nice to know I've been lied to by a "professional"


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## Eric The Viking (10 Nov 2017)

*Thing #1:* Fuses (unlike MCBs) are intended to blow at 2x the rated current, in continuous use. That's unless they are special classes of fuse, which doesn't apply to a hedge trimmer! So a correctly made 13A fuse should blow after a defined time spent passing 26A (too early - can't remember the exact spec: Google it!). 

Start-up surges usually don't affect plugtop fuses (unless the conditions are really marginal), because of the conservative use of numbers, but they DO affect miniature circuit breakers (MCBs), _because those are supposed to switch off at the current shown on the casing_.

*Thing #2:* The two purposes of a fuse in the plug top (officially) are (1.) to protect against fire _in the cable_, and (2.) to protect people against death by electricity. Protection of whatever-it-is against repairable damage would be down to a separate fuse inside the equipment. This means a lot of domestic kit is fused so that the appliance is significantly damaged before the fuse operates. _This is normal._

*Thing #3:* There is an expectation by manufacturers that anything used out of doors will be in a circuit with a RCD (residual current device). This could be at the consumer unit (mandatory for 17th Ed. Regs-compliant installations), or the plug-in type. 

The RCD is very important in these circumstances: much domestic kit is "double insulated", which is arguably the most evil marketing term ever (after "tobacco is good for you"). All it really means is this: _as manufactured, a double-insulated appliance has no earth in its cable, nor its case (if it is a wall-wart type of power supply), even if the earth pin of the plug is metal_. So, if you damage the power cord of a hedge trimmer, it is quite possible that it will stop working, but with live blades (because the cable is caught), _and they will stay live_, as will the damaged cable, until long after you have grabbed that end to try to free it. 

Of course it is likely that the machine will cut both wires, but if it only exposes the live one, the thing will also shock any bystander who tries to prise your fingers off - two casualties for the price of one! In these circumstances, an RCD may not trip initially, but it should switch off _almost_ immediately a live-to-earth circuit is made, either through you or the hedge or the live blade touching the ground, and hopefully will do that fast enough to prevent sufficient joules of energy through the heart muscle to kill it (and the rest of you).

Double insulated supposedly means that any exposed metal in the appliance is doubly-insulated from mains (whatever that means). And yes, people do occasionally die when they go to untangle cables from hedge trimmers, mowers, etc. without unplugging the device. There are fewer incidents nowadays, because of RCDs. 
. . .

So, your hedge trimmer should be fine with a 3A or 5A fuse, and you now know why 13A fuses are often supplied (it doesn't make the thing any less safe, and manufacturers only need stock one sort of fuse). That said, I usually check new kit, and nowadays it often _does_ come with a sensible sized fuse fitted. I'd try 3A to see, but if you use it for hours at a time, consider 5A: The smaller fuse should mean a slightly smaller notch of evaporated metal when you cut through the cable!


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