EBME Forums Forums Maintenance Technical Assistance Moulded Mains Plugs

We take earth-bond readings so that we have the capability of comparing previous readings thus the potential to track changing trends in earth-bond resistance that may indicate the onset of a failure in the integrity of the earth connection don't we? This applies to all the readings we obtain during safety tests, in fact. Assuming 24/0.2mm or 32/0.1mm cables, whatever, for example, having only a couple of strands intact, i.e. only a fraction of its cross-sectional area, the use of a wide range of currents less than 25A should enable indication of an appreciable difference between previous earth-bond resistance readings in a cable to be made where the earth-bond is intact and afterwards when only a few strands remain.

That's Ohms law. We don't necessarily test to "fuse" a poor earth-bond but I believe we must pass enough current to give accurate, sensitive and repeatable earth-bond resistance measurements. It's an undeniable fact that dissipating excessive power into a poor earth-bond connection for long enough is likely to cause it to fail before it would have without a test or fault current being applied. The poorer the bond and the higher the earth-bond test current the more likely this is in my opinion.

25A is probably only required for manufacturing tests, thus 1.5x load current, 10A, 1A or even 200mA may be more appropriate for safety testing after repairs or for routine checks. This depends upon the rationale for applying different standard tests of course. 1A is suggested in DB9801, for acceptance testing, for example. I'm certain that a framework or rationale for testing (possibly in the form of a British standard for safety testing of medical devices) will pop up eventually to cover different scenarios, e.g. post manufacturing, post repair and routine test scenarios. Each with possibly different requirements, thus standard tests, for assuring safety.

The draft British standard on safety testing after servicing (based on the German VDE701 standard?? but I could be mistaken) may have a requirement for earth-bond test currents as low as 200mA I think - for tests that are used routinely to establish on-going equipment safety. The tests will probably not require safety checks to be made under single fault conditions for medical devices either. This will probably speed up routine safety testing and be quite adequate assuming that the more rigourous testing has been performed at manufacture or after equipment has undergone disassembly or major repairs.

The requirement for earth bond testing only states a resistance, which will be the same for any current passed - neglecting heating effects, so any current will measure this resistance.
In a live to earth short circuit fault condition you want the earth to pass enough current to blow the mains fuse thus rendering the equipment safe. So the current rating of the earth will depend on the rating of the mains fuse. You will find mains leads and extension leads rated for lower currents have smaller wires. So passing 25amps down a wire only intended to take 5amps is surely an unfair test. This does not take in to accound people putting the wrong rated fuse in equipment or leads but we ahve to assume the thing is used and maintained correctly.

In my experience the most common cause of failing an earth resistance test is a tarnished earth pin on the plug. If something failed by a small margin a bit of emery cloth soon sorted it out.

Robert

PS Remember the limits for detatchable mains leads and permenantly wired mains leads is different, 200 and 100mohms. Who has their automatic safety testers set up for these two different settings?

Problem is that in standards this is usually stated at a given test current. Even DB9801 states 1A. BS EN 60601-1 states 25A maximum. Low currents can make measurement of smaller changes in resistance more difficult to resolve and more susceptible to errors in measurements - unless 4-wire measurements are undertaken and calibration is assured.

Actually the current rating of the earth is fixed by the standard gauge of wiring used to connect the device not the fuse in my opinion (1mm^2 - 10A, 0.75mm^2 - 6A, etc). The fuse required is rated less than this cable rating but greater than the load, obviously. In BS EN 60601-1 25A or 1.5x the load current is applied at manufacture to check the earth-bond can withstand fault currents - hence the appropriate value fuse should rupture before the earth connection fails.


Is the limit not 100mOhm for detachable and 200mOhm for fixed? There is also an earth-bond of 300mOhm under certain circumstances isn't there?

I thought in might be worth digging this one out again.


Yes, in my experience too, Robert. Remember the thread about I Love Tools? Well, that little bit of Emery paper is always there ready in my tool-bag too!

I tend to use the abrasive rubbing blocks that are available from RS components. Come to think of it the last one I loaned to a Philips engineer for this purpose was never returned to me.....

There is an Irish saying, Richard, that goes like:- "What is the use of calling it a lend when I know I will never see it again?"

Well it wasn't mine anyway - it belonged to the taxpayers.

We want it back then!

Contact Philips Medical Systems then.

The current thinking [ Pun not excused ] from IEC 62353 is Earth bonding measuring current is no longer 1 or 10 or 25 amps, but is 200ma.
This is achieved in the case of Rigel by discharging a capacitor down the earth wire to achieve 8 amps, this burns away the contact resistance, [like one sending down 10 or 25 amps.], then a sensing current of 200ma is sent down, This measures the actual Earth bonding Resistance. Rigel indicates that using less than 8 amps ie 1 amp as per the production inspection line HEI98 scenario causes a high number of false failures for routine PPM.. Using 8 amps causes the resistive film of the plug/ socket to burn off, thus allowing a true reading to be measured at 200mA. Hence small hand held Test Equipments not requiring a 25 amp transformer can be produced to IEC standards.
Try using a plastic Aldi Brillo Pad to clean the earth on a 13 amp plug bit cheaper than RS.
As for changing a moulded plug, if the EBME Dept concerned was outsourced to a 3rd Party Vendor, would the NHS Trust pay the Contractor to change moulded plugs that also have a years warranty from new anyway ?? More chance of seeing a Fairy in a Will of the Wisp??