Member postings for Robert Atkinson 2

Hi Chris,

Can you please explain your reasoning behind the earthing system making a difference to machine leakage current? I think I'm missing something.

Any connection between neutral and earth downstream from an RCD will alost certainly cause it to trip.

Robert.

Clean the first couplr of turns manually. Bore out a block (tufnol, alloy, wood) to the OD of the post. drill two holw onf pitch apart 90 deg to the bore. fit a pin with diameter = groove width in one and a clean-up cutter in the other. run it up the thread with the guide pin behind the cutter.

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OK we anre now deviating away from the original issue of LEAKAGE currents of around 0.03 amps (30 milliamps mA) into Mains earthing systems that have to carry FAULT currents of hundreads of amps. The leakage currents form VFDs and similar devices are not due to faults. A resistance of 10,000 Ohms (10k) with 240V (UK mains) across it will cause a current of 0.024A to flow (Ohms law I = V/R). This close to the 30mA trip current of a domestic RCD. !0K is thousands of times higher resistance than a mains earth. Lets forget about earthing systems.

SOD. The Fluke picture is not helpful becuase it is addressing using a clamp meter for it's typical application of mesuring load current not leakage. I can design a vey simple means of detecting leakage current. A current transformer with two primary windings one carrying the current in the line the other tha current in the neutral. Oh wait that's a current clamp meter around live and neutral. It's also what is built into most electrical installation meters.

Chris P. I said a junction box screwed to wood and exposed to water (a fault condition) not a new, dry box.
The earthing system in use TT, TNC-S etc is irrelevent to this discussion.

As for finding where the leakage is that is not difficult given a sensitive clamp ammeter. First measure and note the leakage current by placing the live and neutral in the jaws of the clamp meter.
To deterine if the leakage is through the desigined "earth" protective conductor add the earth conductor to the L&N in the jaws. Any remaining current is leakage to a connection outside the designed earth system such as damaged insulation.
This process can be carried out at an individual piece of equipment, a particular section of an installation e.g. a ring and the whole installation. Thia allows the exact source of the leakage to be identified. However the fact you would be dealing with individual conductors means that the first layer of protection against shock e.g. equipment case, outer insulation has been removed exposing you to a higher risck of electric shock. This work should only be carried out by competent persons.

As an extension to the testing, it is also possible to tell how much of the leakage current is due to the filters in a VFD or similar. To do this you need a phase sensitive current sensor. As the filter leakage is due to capacitors the current is ninety degrees out of phase with that caused by "normal" resistive leakage.
Note that this intentional current nomally flows through the metal case of the filter as well as any earth wire. You should never earth a filter or VFD purely by bolting it to an earthed metal panel ther must also be a flexible cable connection. This is because if the filter or VFD was unbolted from the panel the leakage current would become a shock risk to anyone touching it.

I've designed this sort of equipment. An example is protection of the high current feeder between a generator and a circuit breaker panel without using fuses in the feeders. You just run the feeder conductor through a current transformer at the generator end and another at the load end. Connect the transformers secondaries in series anti-phase to a sensitive current relay. Load current flows through both transformers and cancel out. Any current due to a fault between the two transformers (to neutral or "earth" only flows through the generator transformer and is not cancelled causing the sensitive relay to operate and shut down the generator.

Robert.

The Guidance seems to imply regulation 4 applies in this case (trader-consumer, digital content, direct debit or credit card. Section 13 says discounts to encourage a particular payment method are allowed but the discount must not exceed the actual dfference in costs of the two payment methods.

I Just got a email about renewing my digital subscription to MEW. The price was given as £48 for a single payment by direct debit. To pay by card or paypal is £52 for the year. How can this be? I thought charging more for card paments was outlawed a few years ago.

It does not have to be leakage to a conductive (metal) part extraneous or otherwise. An old junnction box screwed to wood and exposed to water will cause leakage current. The RCD senses line / neutral differential current so that is what you need to measure when testing.
A mill bolted to a even slightly damp concrete floor would provide a parallel path to earth.

Wot I said....

Strangely enough a "cheap chinese" VFD is, barring faults, likely to have less leakage current than a good quaility one. This is becase the cheap ones tend to leave out the EMC fiter capacitors that cause the leakage current.

You don't actually measure thr current in the CPC ("earth" conductor). This is becuse the leakage culd be finding a different path to ground. You do what an RCD does, measure the difference in current between the line and neutral. A clamp meterto do this has to go around the line and neutral.

Robert.

This fault is as already been said almost certainly caused by leakage currents that result from the design and normal operation of the VFD and filers in particular. This higlights the fact that almost all VFDs are industrial devices and not intended to be connected to the domestic mains supply.
Disconnecting the filter is not the correct solution.
Ideally the workshop would have it's own dedicated feed with an isolator (main switch) and circuit breaker (no RCD) at the incoming supply conected to a consumer unit in the workshop with second isolator ,a RCBO for each machine and additional RCBOs for ring main and lighting.

Robert.

I found this thread interesting as I'm just doing a job that needs a flat surface with a good finish. It is similar to Andrew's application a heatsink for high power semiconductors. This is a aluminium heatsink and as usual with these it is near pure so soft and sticky. It's also used so I have to re-surface it and plug a couple of holes. The original finish was obviously made with a fly cutter. I did consider that approach but my mill is a small high speed model (SX1LP). so not Ideal. I decided to try a 90 degree (two insert) 25mm indexible end / side mill from ARC
https://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Indexable-Carbide-End-Mills/90-Indexable-Carbide-End-Mills
with APKT 1604 PDFR polished inserts. This has the added advantage that the semiconductor (MOSFET in SOT227B package) will just fit in the width of a single cut.
Had a go today. Using it dry with vacuum chip extraction, 2100 RPM and light cuts. Finsh is pretty good (not up to Andrews standard). It's clearly a milled surface but no roughness detectable by a fingernail .A check with some blue, a flat and one of the devices shows it is flatter than the MOSFET. The critical area of the MOSFET is in good contact with just finger pressure dispite not being perfectly flat. I'll sort out some photos. The holes that needed plugging were tapped M3 so I put plated brass screws in with Loctite 601 retainer. Left it for an hour and just machined the heads off.

These supposed "PWM" controllers like this one:
https://www.amazon.co.uk/DollaTek-Control-Controller-Adjustable-Regulator/dp/B07F9L5557/ref=sr_1_5?crid=T2GZU4OP4J8F

Are not PWM they are phase angle controllers. They control the average power. At anything over 50% the peak voltage will be the same as the peak mains voltage. They are OK for heaters, incandescent lamps and universal motors. They will work, as previously noted, with fans due to their load charactistics.
The grinder is probably usable due to it's large inertia. I bet you have to staart it ax maximum and then bring the speed down.
These items are components, not equipment and need enclosures, fuses, filters etc. In thery they also need to be tested for safety and EMC. I can say for certain thet the one linked to will not pass EMC testing.

There are also more "packaged" versions like :

https://www.amazon.co.uk/Regulator-Controller-Variable-Electric-Temperature/dp/B0BYSX8QBT/ref=sr_1_18?crid=T2GZU4OP4J8F

But it clearly does not meet UK safety standards and the plug is unsafe.

Robert.

A pocket sized 50mm angle grinder - bike thieves must love them.

Unmarked kerosine is sold at the full rate of duty.

Robert.

Chopping boards are generally made from the Ultra high molecular weight polythene (UHMWPE) mentioned by Macolm. Also useful as neutron moderators

Robert.

Edited By Robert Atkinson 2 on 22/09/2023 16:58:44

Correct. The problem with Aviation fuel is that it is not marked (dyed) so they can't easilly tell if it is used in a road vehicle (highest taxation class). This means there are stricter regulations and controls on it.
The reason it is unmarked is because the marker could interfere with correct operation of the aircraft systems. Even if it didn't the specifications for the fuel do not allow the addition of markers.

The laws were changed fairly recently with more restrictions on reduced duty fuel. You can no longer use it in transport refrigeration units and pleasure boats can only claim a rebate for the portion of fuel used for heating rather than propulsion.

Robert.

NO NO, Hell NO!!!!!

These chinese diesel heaters are not well made, are not intended for use in dwellings and are not approved for this type of use. Most of them are not properly approved for ANY use. If it burns the house down you will probably find you insurance was invalidated by usin one.

Additionally Steve, assuming that the "kerosine" you bought at the flying club is aviation fuel, you have just admittedt to fuel duty fraud on a public forum. I can't see why you would by any other kerosine from a flying club. It is also illegal for them to sell it to you.

In contrast I buy Jet-A for my gas turbine engine from a responsible supplier and have to pay the full rate of duty. In theory as I'm using it to test an aircraft engine it should be duty free but the supplier, quite reasonably in my view, say that ANY sale that does not go into a aircraft or ground support equipment on the airfield is charged at full duty. The small number of sales is not worth the effort to ensure that it is not duty evasion given the consequences to their business if it is. If I don't like It I can go elsewhere as far as they are concerned.

Robert.

Hmm, those appear to be 3 Phase output, single phase input VFDs. The OP needs a single phase OUTPUT VFD.

+1 on Tony's comment on using immersion elements without water. They are designed for fast heating so need the water to control the temperature.

Another issue is that solar cells don't work well with fixed resistance loads. For your sand heaters idea you could use some MOSFETs on a heatsink in the sand and a maximum power point tracking (MPPT) controller.
BUT
You would probably be better off using the solar cells to charge a 12 or 24V battery and then running a car or truck fan heater off the batttery when you actually need heat.

Robert.

Edited By Robert Atkinson 2 on 22/09/2023 12:44:55

You can get single phase VFDs but they are uncommon. This makes them relatively expensive. Most are not suitable for capacitor or centrifugal start motors. Proper details of the motor would be required before anything else can be decided. Are you sure it's not a universal (brushed) motor? that would open up a lot of possibilities. They are common on machines with the motor integrated into them.
Reducing the speed will also reduce the available power.
Unless it is a particuarly special machine it's probably not worth the effort.
Why can't you change the motor to a 3 phase one? As I note above if it's highly integrated it may be a universal.
Just because someone on on the internet says you can use a 3 phse one does not make it so. The output would be severly inbalanced and would, or should trip the protections.

Robert.