Resistor voltage rating

Den torsdag den 30. april 2015 kl. 18.43.20 UTC+2 skrev John Larkin:

it not a company name that they care about, they want a VAT number

-LAsse

You've been in business for awhile--you don't have any pals with VAT numbers?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Do you need a VAT number to buy parts?

We need to furnish some sort of number to a California distributor if we want to avoid sales tax, namely if the related product is "for resale." If the parts are for consumption within our company (like supplies or test equipment) then they cheerfully sell to us, but include the roughly 8% sales tax. If we buy from out of state, for consumption, we are usually responsible for reporting the sale and paying the California sales tax.

VAT sounds complex.

We charge our customers sales tax on tangibles if they are in California, and are not a government agency, and do not want the gadget for resale. The combination is rare.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

California is weird about sales tax. When my California customers need protos built, they buy the parts and send them to me, which short-circuits the whole mess.

For small stuff, like electronics parts, I usually buy them (and pay NY sales tax) and then just get reimbursed.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Den torsdag den 30. april 2015 kl. 21.43.57 UTC+2 skrev John Larkin:

not if I buy from the numerous shop that deal with regular private consumers but if I want to buy from Farnell/RS/etc I do since they only deal with businesses

not really, when a business buys parts it gets the VAT back, in return it has to add and pay in VAT on the sales price when they gadget they made with those parts is sold

-Lasse

I have a genuine registered company, have had it for 40 years now. But the registration is for official recognition as an industry contributing to progress and development rather than as a profit-making trade. There's a nominal annual fee and nobody cares if I don't pay the dues as long as I don't apply for some government aid. Any form of taxation was introduced much much later in my state and VAT registration is an entirely separate matter.

You describe the scenario very well. It's the same here except that it's made worse by a combination of ineptitude and corruption.

Agaun, the same principles apply here. But then, the side topic is about the need to have a VAT registration to buy from certain sources.

You took the words right out of my mouth. I'm in the same situation except that my location adds many other complications.

That's the theory and maybe that's all there is to it in some places, but not where I live. Our tax people lack experience and perspective. All they think about are merchants who buy finished products and resell them at a profit.

Not very hard. It's the hassle forever after that puts me off.

Not everyone is interested in running a "business", strange as that may seem to some. Personally, I'm a pro in some ways but a hobbyist at heart. I've had opportunities to make some real money but I passed them up, sometimes reluctantly, for other priorities.

I have nothing against making money. But at the end of the day, I'd rather be a moderately comfortable hobbyist than be bored to death running a lucrative business. I've put three kids through college - two engineers and a nursing graduate who's starting her master's in August. The fourth and youngest is just finishing his

2nd year in architecture. Oh, I'd love to be filthy rich but one can't have everything.

Yes, I do. One of them is a former bank clerk who resigned from his job to take up electronics under my tutelage.

But let's not lose track here. The VAT thing is only a small part of the problem and only came up in the thread when I mentioned Farnell and RS. The main problem is that it's well nigh impossible to deal with Indian retail sources from a distance unless it involves regular high-volume orders. OTC sales are OK but that requires living in or making a trip to one of the major cities.

Hey, we have stupid tax people here, too.

We don't pay sales tax on parts that go into sellable products, only on parts that we "use". One California tax guy claimed that, when we test something, we are "using" it.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Den fredag den 1. maj 2015 kl. 00.37.09 UTC+2 skrev John Larkin:

it can happen here too, i.e. if a guy with a small company in his home office buys a new computer and TV and doesn't have a private computer and TV the tax man will assume that it is for private use and he can't deduct it

-Lasse

That's normal; home office stuff here has to be exclusive. I could deduct my computer stuff, since I do a lot of work on it (and even conversations like this could arguably be considered business development...ha, yeah right?)... but the Steam games (and others) also on it would kind of compromise that claim. ;-)

On the other hand, stuff like test equipment and electronics parts don't need to be "for hobby use"; that's direct research right there!

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com

The nice things about a business, aside from getting paid, are

You get to work with other people

You can afford more toys

Customers present interesting challenges

Sometimes when you do good, important customers say so. It may sound shallow, but the best validation that you are doing good work is when a big company says so. And buys more.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Saying nice things is cheap. Buying more product is a more sincere form of admiration, and harder to fake.

The nice thing about business - as opposed to academic mutual admiration so cieties - is that self-deceiving nitwits lose money and go out of business.

There's not much evidence that mutual back-scratching makes much difference to citation rates in academic circles, but it's cheaper to cite a dodgy pa per by a friend than it is to buy their over-rated hardware.

John Larkin's business model isn't quite as commercial as he likes to make out, but the survival of his business is a more reliable indicator of quali ty than the 19 citations for my most cited paper - for a start two the cita tions are mine, and two more are in Russian

--
Bill Sloman, Sydney

Hi

There is a lot of information regarding safety isolation of test and measurement instruments in the relevant standards - like IEC61010.

The standards documents are usually expensive and in your neck of the woods may be difficult to obtain, but you can sometimes find a copy of a "draft" or "voting draft" version on the net. Often these end up sitting in the depths of the web servers of various agencies that were involved in the development of these drafts. Here's an old FDIS of the 61010 that Google has found on the ISA website:

Note that this (or any other) FDIS may not correspond exactly to the published standard - so it should not be used as reference when you intend to obtain agency certification for a product. But for your own use, when you care about your own safety and not about a stamp on a piece of paper, even a draft standard - used together with healthy common sense and good engineering practices - can still go a long way.

Since I (and, assumed, you also) don't have a copy of the official standard, I'll refer to chapter numbers of the document from the ISA website above. Their names and exact contents may have changed, but I think that they're still a good start.

For once, when it comes to electrical safety, there exists a generic concept of the number of "means of protection".

If the component that constitutes an insulation barrier (your resistor) is situated between mains phases, but it has no connections to your body (to conductive parts of you instrument that you can touch), it will require at least ONE means of protection.

If the component is situated between a line from the mains and YOU (e.g. its other end goes to a low-voltage circuit that connects to a metal part that is part of the device housing that you can touch), then this component is required to have TWO means of protection.

The reasoning is, that if one of those means of protection should become defective and fail for any reason, you would still be safe.

To achieve ONE means of protection, the part needs to provide at least BASIC INSULATION, to achieve TWO means of protection the part must provide DOUBLE or REINFORCED INSULATION (or alternatively two means of protection can be achieved by using basic insulation in combination with protective bonding).

In addition to that, the insulation must be able to survive, without being compromised, the failure of any single part. That means, if ONE resistor gets shorted (in any way and for any reason), the required BASIC or REINFORCED insulation must still be intact. This means that you should always use a series string of resistors and size and rate that string so that if ANY ONE of these resistors suddenly disappears and is replaced by a short, the remaining resistors (together) still have to provide the required degree of insulation (that is, use ONE more resistor than would otherwise be needed).

There is an exception to this rule, see chapter 14.6 "High integrity components" (with your typical cheap resistors of unknown provenance that won't apply to your case).

Ok, now with this basic idea in mind, and assuming that none of your available components are rated as "high integrity", you have to decide what type of insulation (BASIC or DOUBLE / REINFORCED) applies to you.

If your resistor string is - between the phases - between any phase and neutral - between any phase and protective earth, apply BASIC INSULATION.

If your resistor string is between any phase and an ACCESSIBLE PART (any part or connection that can potentially contact your or someone else's body), like - unearthed metallic housing - unisolated controls, buttons, displays - conductive rubber parts, coatings - accessible bolts, screws, mounting hardware - etc. then apply DOUBLE or REINFORCED INSULATION, in addition, if you use any components (resistors or not) in this way, see also ANNEX A and chapter

6.3 on the PERMISSIBLE LIMITS FOR ACCESSIBLE PARTS, especially "6.3.1 part b) part 1)" for normal operation and "6.3.2 part b) part 1)" in single fault conditions. For resistors also refer to chapter 6.5.3.

For an overview of the clearances and creepage distances see chapter 6.7 (it makes sense to read this chapter completely).

Having figured out, which type of insulation you need, decide, in which type of environment is your device to be used, according to chapter

6.7.4 and therefore which MEASUREMENT CATEGORY applies to it.

If you are not sure how to interpret that, then - for any connection BEFORE the "house fuse box", use MEASUREMENT CATEGORY IV - for a connection to fused circuits, when you don't know where and how permanent that connection will be made, use MEASUREMENT CATEGORY III - Since you are building a 3 phase device that is unlikely to be a "household appliance", category II won't likely apply. - For any type of mains connected circuits, category I does not apply.

As a guideline, use the overview figure 2a at the beginning of chapter 6.7.

See table 4 in chapter 6.7.2 and table 8 in 6.7.4 for minimum clearances according to measurement category and nominal line voltage, also see table 3 if you live in the mountains. Use the higher applicable value. Use table 7 for the creepage distance for basic insulation, double that for double / reinforced insulation.

Finally, make sure you rate your resistors for TWICE the power dissipation (see 14.6) at the rated mains voltage and make sure they (the whole series string) can withstand an impulse up to that of table 9 in chapter 6.8.4 (peak voltage). Additionally, if the resistors are between mains and an accessible part (need double isolation), multiply by a factor of 1.6 according to 6.8.4.

This means that you will probably need at least 4 (with high transient ratings) or more likely about 6 (typical 1 W) resistors for use across a "double insulation", and about 3 (with high transient ratings) to 5 (typical 1 W) resistors in series between the phases (across a "basic insulation"). That's assuming category III.

Please note that this is only an estimate, not intended to provide ay kind of advice regarding any type of guaranteed safety. Besides, it's based on an old draft of a standard with no legal validity. Therefore, if not sure, err on the side of caution and when in doubt, apply common sense.

Dimitrij

I think the only way you're going to know what will survive is to test a few of these Rs to see what voltage they arc over & die. I'm interested in the reuslts if you do.

NT

Others will probably be interested too. But there are two problems with that. One is that most GP 1W resistors are rated for 500V continuous and 1000V intermittent. Even 0.5W types are rated for 350/700V. And they probably won't blow up immediately when these values are exceeded slightly. I'd have to rig up an adjustable high voltage source to see at what voltage they do arc over.

The second problem is that resistors from different manufacturers, different batches and even individual specimens from the same batch will probably not break down at the same voltage. So, like you said, I'd have to kill several specimens to come to a meaningful conclusion. And all to use a few resistors in a gadget that's going to lie idle most of the time.

What I was hoping for was to draw from the collective wisdom and experience of the group.

I assume they don't want the hassle of dealing with hobbyists. but the barrier is still fairly low, just high enough to push those who can be coerced to other suppliers like "sparkfun". it's not like they audit you for proof that you are doing business.

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umop apisdn