Re: Surge Suppressors
Hi Ray: In Norway, there is no guarantee that any particular outlet will include a ground. At NEMKO's main meeting room, the outlets do not have a ground! Consequently, in Norway, the condition of no ground is a NORMAL condition, not a fault condition. (Likewise, one-third of USA homes have no ground, and all Japanese homes have no ground!) So, Norway prohibits surge suppressors from being connected to 'earthed' parts of equipment. Now that we've put that issue to bed, let's look at another issue: Does a shock hazard exist because a surge suppressor is connected to grounded parts of equipment when the equipment is not connected to ground? Let's first define the surge. Can we assume the "standard" 1.5 x 50 impulse? If so, we are looking at a "surge" that decays to 63% of its peak value in 50 microseconds. It decays to nearly zero in 250 microseconds. We can further define the surge amplitude as not exceeding 1.5 kV. This is the dielectric withstand value of the primary-to- ground insulation. Let's assume the surge is less than 1.5 kV peak for less than 250 microseconds. Is this hazardous? Can the human body feel this pulse? Probably. Can the human body be "frozen" to the product by this pulse? No. "Freezing" requires continuous current through the body. Can the heart be caused to fibrillate by this pulse? No. Fibrillation requires the current to extend for the full duration of the T wave, i.e., more than 200 milliseconds. Can the human body be burned by this pulse. Yes. A current of 70 mA peak or more can cause a burn at 1 cm square contacts with the skin. It is likely that the body impedance will be less than 1500/0.070 = 21,428 ohms during this pulse. Note that protection against electric shock by the GFCI (aka ELCB, RCCB, RCD) provides protection by limiting the duration of the current through the body, not by limiting either the voltage or the current. When a GFCI operates, the subject gets the full voltage and whatever current, but only for a short period of time. In this way, it prevents fibrillation and "freezing" to the circuit. So, I submit that the impulse, while it can be felt and may create a burn, is not a shock hazard. Now that we've looked at electric shock, let's look at whether the impulse can appear on the grounded parts of the equipment when the equipment is not grounded. Ignoring the surge suppressors, and considering for a moment the EMC filter capacitors, we have the following circuit: L1 <...-+-+ | | | | ---+--- | CY1| ---+---- | | | Rload +---+ | | | | | | ---+---| - CY2 | | ---+---| | | | | | | | | | | L2 or N <...-+---)-+ | PE (non-existent) <...+ | | chassis - --- - If a surge appears between L1 and N, it will be divided in half by the action of CY1 and CY2. So, only one-half of the surge voltage will appear on the chassis. If a surge appears common-mode between L1/N and PE, then the full voltage will appear on the chassis because there is no current path (because the ground is open). We can replace CY1 and CY2 with surge suppressors. If a surge appears between L1 and N, the surge suppressors will not be turned on unless the suppressor voltage ratings are less than one-half the applied voltage. In this case, the circuit response to the surge remains the same as with the capacitors. If the suppressors are not turned on, then the circuit still behaves the same as with capacitors because of the capacitance of the suppressors. If a surge appears common mode, the surge suppressors will not be turned on because there is no connection of the chassis to any other circuit. The full surge voltage will appear on the chassis just as with the capacitors. I submit that: 1) There is no electric shock injury hazard from a surge, but there may be a burn hazard. 2) If the unit incorporates surge suppressors and is not grounded, the hazard is no worse than without the surge suppressors. Best regards fr
re: US Institute???
Mike: By any chance could the U.S. Institute be ANSI - American National Standards Institute ? Richard Payne Tektronix Inc. richard.pa...@tek.com
US Institute???
Hi; Has anybody heard of such a thing as the "US Institute"??? I am not sure "US" refers to "United States of America." Somebody in South Africa just whacked my boss with "we only buy products that satisfy the US Institute Protocols". Naturally, he "managed" the situation by promising an immediate answer, then dumping the whole thing on my head. *sigh* Any and all help will be greatly appreciated. Cheers! Mike Mertinooke Mike Mertinooke
Uncertainty calculations
I have recently started looking into the uncertainty associated with EMC measurements, I have three documents published in the UK, NIS 80, 81 and 3003 which cover the topic, but I would like to see any other documents or examples of calculations that are available. Thanks in advance for your help Peter. -- *** Peter Phillips (p.phill...@ftel.co.uk) Fujitsu Telecommunications Europe Limited Birmingham B37 7YU England Telephone +44 (0)121 717 6115 Fax +44 (0)121 717 6014/6018 ***
Surge Suppressors
Dear Friends, It seems that surge suppressers have been a topic of much discussion lately. Must be all of the surge testing to EN61000 4-5! Well at least that's what brought up our issue. We have a piece of 230V equipment that failed the surge test, (safety testing will be to EN61010). In reviewing how to install varistors, we have run into some concerns. In Europe the 230V supply is 1 line, 1 neutral, and 1 ground wire. A varistor connected between line and neutral will suppress a surge, as long as the neutral is grounded. For the 230V US voltage, there is 2 hot lines and a ground. To make the varistor effective, it seems that the varistor needs to be connected to ground. However, I know of a Nordic deviation out of EN60950 that states: (DK, FI, NO, SE). Transient protection components shall be installed in such a way that insulation for protection against electric shock will not be bridged. This means that transient protection components must not be connected to protective earthed parts in pluggable equipment or to other accessible parts. I think the concern here is the possibility of not having a good ground, and then if the over voltage is shunted to ground, the chassis could become hot. Is this a valid concern? It sounds like a "double fault" scenario to me. If so, does anyone have an alternative solution? Thank you for your consideration. Happy is the man that finds wisdom and gets understanding! Ray Russell ray_russ...@leco.com
An Irrational Guide to Electrical Safety and the LVD.
Friends, I couldn't resist sharing with you this emanation from the fertile brain of our good friend Gregg Kervill: An Irrational Guide to Electrical Safety and the LVD. Release date April 1st. The Low Voltage Directive We will start by taking a top-level look at the Low Voltage Directive (LVD) and some of its implications: Considerations The LVD has been about since 1973: therefore after more than 24 years we know all there is about it and can therefore safety ignore it. Alternatively we may consider that there are already a significant number of people killed by electrocution and that if our product kills only a few more that it wouldn't make much difference - statistically speaking - that is. The CE Mark Products within the scope of the LVD must be marked with the CE Mark. The committed Europhile will have included the additional very tiny letters "a-v-e-a-t" and "m-p-t-o-r" amongst the letters C and E to produce that universal warning to all of his customers. (Viz. "Caveat Emptor" = Let him beware.) The advantage of using a "dead" language for warnings is that will be universally unrecognised by the very people that they are intended to help - but will favour the elite and educated and hence protect our own "Kith and Kin". The Responsible Person Always appoint the least technically competent employee for the task of Responsible Person. It is equally important to ensure that they can be easily intimidated; will help suppress concerns that a potential non-compliant product has been in production for the last 15 years; and that they will continue to sign Declarations of Conformity despite a series of memos and reports for the quality department and other conscientious employees. The "Data File" The "data file", legally required by the LVD is no problem to the seriously minded. True the requirements to hold information that clearly (and publicly) identifies non-compliances against European Harmonised standards could be seen as a 'bit negative' (in marketing terms) but a careful review of the many hundred of harmonised standards should yield a few to which we can claim compliance - even if they relate to the pulp content of our user manuals and not to the product itself. It is well known that there is no need to review, test or consider any safety features of products outside the scope of the LVD - as it is perfectly reasonable to place potentially dangerous products on the market: provided that they are not CE Marked. Being Sued Only the 'wimps' worry that they may be sued by people injured by their products. The "real-men" take confidence in their design capabilities. If the product is not compromised to pander to the requirements of product safety - life is beautifully simple: there will be no survivors and therefore no-one to sue. Hence there is no problem. Quod Erat Demonstrandum! Furthermore the lack of customers is nothing to do with engineering and the blame can be placed squarely at the door of the marketing and sales departments. A Look in Detail Next we will take a look at some of the more detailed requirements contained in some of the widely used Harmonised Standards - the Eight Principles relating to Hazards. Knowledge: Avoid training and employ only junior staff - this will ensure that product safety is NOT consider early in the design and therefore will save huge amounts of money. Having spent the entire budget we can then bully the Responsible Person (see above) into signing the release documentation. Electric Shock Committing the purchasing department to buying critical components (e.g. power supplies) that carry an agency approval is restrictive. The purchasing department must be given total freedom to buy from the cheapest - third-world sources. We all know that the final selling price is the MOST important thing to achieve. Energy Hazards It has been scientifically proven that subjecting the Operator to circuits delivering more than 8 Amps, 240 VA, 60 Volts or 20 Joules keeps them more vigilant and they work harder. True there may be a few deaths or injuries caused by these energies but these are usually restricted to the old, ill or infirm. It's nothing to do with us or with our product. If we are really clever and have a 48 Volt supply from 200 kg of lead acid batteries we may be able to get our customer to create the hazard. All we do is to over-emphasise the fact that the operator outputs are Safety Extra Low Voltage (SELV). Most of our clients will be unaware that only Safety Extra Low Voltage Energy Limited (SELVEL) may be contacted by the operator: and it shouldn't be long before someone discovers that the fault current in these external circuits could be more than 2,000 Amps. Fire Use the thinnest section of plastic that it is possible to mould. Make sure that the purchasing department specify only the colour of the plastic - this will usually guarantee that the cheapest - most flammable materials are sourced. Avoid meta
machinery directive forum / list server
Hello everyone, does anyone know about a an information source/list server specialising in the machinery directive? Do not be offended, but I did receive a considerable amount of information re ergonomics from a (non emc-pstc) guy who works with the machinery directive. No one referred to this info on the "emc-pstc". I will prepare a summary ASAP of course. thanks - Name: moshe valdman E-mail: mvald...@netvision.net.il Phone: 052-941200, 03-5496369 List-Post: emc-pstc@listserv.ieee.org Date: 13/05/97 Time: 0:00:51 You are most welcome to visit my homepage at: http://www.geocities.com/CapeCanaveral/5233/ -
