Kenwood Chef Mixer - Metal chassis but no earth in flex

[aesmith]

Hi,
Our Kenwood Chef A901P is in bits waiting for me to fix the speed control circuit. Looking at other bits the flex looks a bit rough so I might replace that. The original is two cord, is there any reason not to replace it with three core and fit the earth to some part of the metal chassis? Just looking at the flex it looks possible that with a bit more damage one of the conductors could have touched the metalwork.
Thanks,

 

[ericmark]

Class II equipment. Equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions such as supplementary insulation are provided, there being no provision for the connection of exposed metalwork of the equipment to a protective conductor, and no reliance upon precautions to be taken in the fixed wiring of the installation (see BS EN 61140)

Not sure if permitted to add an earth?

 

[aesmith]

Cheers. What would be the danger from adding one?
I should say I'm not completely set on it, just worth a consideration if I replace the flex. The existing was damaged by being pulled through the strain relief which stripped off the sheath and damaged but did not penetrate the insulation. I can bodge by loosening the clamp, pull more flex inside the appliance before re-tightening so it's gripping the sheath.

 

[JohnW2]

Our Kenwood Chef A901P is in bits waiting for me to fix the speed control circuit. Looking at other bits the flex looks a bit rough so I might replace that. The original is two cord, is there any reason not to replace it with three core and fit the earth to some part of the metal chassis? Just looking at the flex it looks possible that with a bit more damage one of the conductors could have touched the metalwork.

As eric has implied, it's probably a "Class II' appliance, which does not require an earth (users being protected by insulation). Does it bear a Class II/'double insulated' symbol (a square within a square)?

Will any part of the metal chassis be exposed/touchable once you have re-assembled the mixer?

Kind Regards, John

 

[aesmith]

The metal chassis is most definitely touchable, it forms the base where you fit the bowl, the bit that pivots up and holds the motor and the control knob. I would also include the gearbox as that's metal cased and bolts onto that second part, so potentially that might extend to the attachment as well since the connection to the gearbox is metal to metal.

I'll need to look for the symbol. The mixer dates from 1982, did they have double insulated classification then?

 

[aesmith]

Some quick photos. First the metal chassis, the parts that are exposed and handled when the mixer is fully assembled.

The motor wiring, I must say I don't really understand why this would be "double" insulated but then I don't know how that term is defined. Note the exploded capacitors and cooked resistor by the way.

When fully assembled that wiring is covered by a plastic shroud, so when the mixer head is tilted up, those uninsulated leads and soldered connectors aren't finger touchable (is this shroud the "double"?)

Rating plate, and yes it does have that symbol ..

 

[JohnW2]

The metal chassis is most definitely touchable, it forms the base where you fit the bowl, the bit that pivots up and holds the motor and the control knob. I would also include the gearbox as that's metal cased and bolts onto that second part, so potentially that might extend to the attachment as well since the connection to the gearbox is metal to metal.

OK - see below.

I'll need to look for the symbol. The mixer dates from 1982, did they have double insulated classification then?

Ours is a fair bit older than yours - early 70s, if not earlier, and does bear the Double-Insulated symbol - but I see that you've now found the symbol on yours.

The motor wiring, I must say I don't really understand why this would be "double" insulated but then I don't know how that term is defined. Note the exploded capacitors and cooked resistor by the way. ...... When fully assembled that wiring is covered by a plastic shroud, so when the mixer head is tilted up, those uninsulated leads and soldered connectors aren't finger touchable (is this shroud the "double"?)

"Double insulated" refers to the fact that there are two layers of insulation (OR one layer of 'reinforced insulation', whatever that may be!) between (electrically) live parts and any touchable metal.

Cheers. What would be the danger from adding one?

It is clear, in both yours and my (older) one, that the mixer is designed and manufactured to be Class II/Double-Insulated, and hence 'safe' without any earthing, that achieved by the double (or reinforced) insulation between live parts and any touchable metal - i.e. any 'touchable metal' is electrically 'floating', not in electrical continuity with anything else.

Given that, the only advantage of earthing the chassis would be to provide you with protection should there simultaneously be an electricidal fault within it and a failure of the 'double insulation' - an incredibly unlikely occurrence.

As for 'any danger' (of earthing the chassis) , it is again an extremely small 'risk' we are talking about. As a general truth, it is desirable that one should avoid having touchable metal which is 'unnecessarily earthed'. The reason for that is if one were to touch something else live (like a metal kettle that had become live due to a fault) and simultaneously touched the 'unnecessarily earthed' metal, then one would get an electric shock, whereas one would get little or no shock if the metal one touched was not earthed (i.e. 'electrically floating'). However, as said, that is also an incredibly unlikely occurrence.

For what it's worth, I would personally trust the design and manufacture of the mixer and leave it unearthed.

Kind Regards, John

 

[aesmith]

You're probably correct. The risk I was thinking of was that damaged flex, passing through the metal chassis, although it would also need to pull the plastic clamp out as well before a conductor could touch the metal.

 

[JohnW2]

You're probably correct. The risk I was thinking of was that damaged flex, passing through the metal chassis, although it would also need to pull the plastic clamp out as well before a conductor could touch the metal.

I agree. It should be designed such that no 'single fault' could result in anything live coming into contact with 'touchable metal' (or anything in electrical continuity with touchable metal).

Kin Regards, John

 

[jacko555]

Maybe helpful:

I replaced the same circuit in wifeys chef, along with brushes and a general service.

One thing that foxed me for a few mins, after taking it apart, was it wouldn't turn on again.

The screws in your picture 2, have springs under, and tightening them moves the board closet to the motor brushes.

So if it doesn't turn on after your work, try adjusting these.

Also, get some new rubber feet. Important for airflow. Only a few quid of fleabay

 

[aesmith]

Thanks. Parts on order are all the speed control components, brushes, and feet. When we'll get these is another matter, not only all the Bank Holidays but the fact that our drive is likely to be impassible to normal vehicles until we have a bit more of a thaw.

 

[plugwash]

OK - see below.

Ours is a fair bit older than yours - early 70s, if not earlier, and does bear the Double-Insulated symbol - but I see that you've now found the symbol on yours.

"Double insulated" refers to the fact that there are two layers of insulation

In *very* simplistic terms yes.

(OR one layer of 'reinforced insulation', whatever that may be!

As I understand it.

Reinforced insulation is a single layer of insulation that provides equivilent protection to the combination of basic insulation and supplementaty insulation. IIRC electrically that normally means withstanding 1.5kV for basic or supplementary insulation and 3kV for reinforced insulaion.

It really doesn't take very much good quality homegenous plastic to meet the electrical strength requirements for reinforced insulation and IIRC standards do not explicitly specificy a minium thickness, but electrical strength is not the only thing that must be considered when determining an insulation barriers suitability. Faults and wear and tear must also be considered.

Air gaps can, and often do, provide insulation barriers, including "reinforced insulation". An air gap used as insulation is known as "clearance", while a path along a surface is known as "creepage", standards set minimums for these depending on the type of insulation. Distances for "creepage" are longer than those for clearance because surfaces almost inevitablly get contaminated and this can result in the path along the surface having a lower breakdown voltage than either the homogenous solid insulator or the open air. This is why you will often see slots in circuit boards that handle high voltages.

However, even with large air gaps, wiring terminated directly to components/terminals inside a metal enclosure without any intermediate barriers would normally be considered to only provide "basic insulation" between the wiring and enclosure, because of the risk of faults where the wiring comes loose and touches the enclosure, or faults where the enclosure gets deformed reducing the clearances.

(is this shroud the "double"?)

Essentially yes,

By putting a plastic shroud between the electrical connections and the touchable metal parts, the manufacturer ensures that even if a wire comes loose, it doesn't touch the metal case and cause a dangerous condition.

 

[JohnW2]

As I understand it. ... Reinforced insulation is a single layer of insulation that provides equivilent protection to the combination of basic insulation and supplementaty insulation.

I'm sure that is conceptually correct, but I don't think anyone here has ever been able to ascertain what actually qualifies as "reinforced" in this context.

IIRC electrically that normally means withstanding 1.5kV for basic or supplementary insulation and 3kV for reinforced insulaion.

I'm not sure that the electrical properties are particularly relevant since, as you go on to say, virtually any credible thickness of almost any any 'plastic' (and a good few other things) would be more than adequate. I personally wouldn't see the point in requiring it to withstand an even higher voltage, given that even basic insulation is required to be able to withstand voltages well in excess of any that would ever be present within the equipment.

It really doesn't take very much good quality homegenous plastic to meet the electrical strength requirements for reinforced insulation and IIRC standards do not explicitly specificy a minium thickness, but electrical strength is not the only thing that must be considered when determining an insulation barriers suitability. Faults and wear and tear must also be considered.

Indeed, given that the 'reinforced insulation" represents the only protection against touching a live part, and given that (as above0 even basic insulation is required to be able to withstand voltages well above any present, I would have thought that the "reinforced" will relate almost entirely to the mechanical strength (in a wide sense) of the insulation - since, if it breaks or 'wears out', the electrical properties (which it had when intact) become irrelevant.

Kind Regards, John

 

[JohnD]

The flex passes through a hole in the metal casing, and is prone to chafing.

 

[aesmith]

Essentially yes,

By putting a plastic shroud between the electrical connections and the touchable metal parts, the manufacturer ensures that even if a wire comes loose, it doesn't touch the metal case and cause a dangerous condition.

See my second picture, if one of those soldered connections came apart (unlikely I know) then the shroud does nothing to stop the loose wire touching the upper part of the chassis. I think all the shroud achieves is preventing fingers from touching those uninsulated conductors when the mixer is tilted up.