EBME Forums Forums Management Medical Equipment Management Safety testing normal & reverse polarity

Lindsay
Measurement of earth leakage current, in normal condition, earth is disconnected and replaced by an ammeter. In single fault condition normal polarity, the neutral is also disconnected.
A neutral/ earth short will not give a high leakage current in normal polarity because you are measuring current between live and earth only.

Regards
Sean

Sean

I might be missing somthing, but in normal condition testing, with a neutral to Earth short, the ammeter (in place of the earth) will measure the majority of the load current of the equipment. The fault would be discovered before advancing to SFC testing, and an open Neutral.

Has 25+ years of my understanding of Electrical safety testing been based on a false understanding?

Lindsay

I'm no expert at safety testing so please feel free to pick this simplistic reasoning to bits but I think about it this way: The aim of routine safety testing, on acceptance, post-repair, whenever, is to maintain patient/operator safety under normal operating conditions and in the event of a likely single fault condition (SFC) that may occur.

For Class 1 medical equipment (with protective earth) under normal conditions; in the event that there is a degradation in insulation between L (at mains potential) and E (close to Neutral potential under normal conditions - remember Earth is bonded to Neutral at the substation) then unacceptably high earth leakage currents will flow. Complete brakdown L-E will result in the equipment protective fuse(s) in the L circuit blowing under normal conditions.

If the equipment wiring is reversed e.g. somebody reverses the L/N connections in the IUT - a SFC exists and the Neutral circuit in the equipment is raised to mains potential (it usually never achieves mains potential under normal conditions to do this the safety tester seperates N from E and reverses L/N). If there is excessive earth leakage under these conditions then there is degradation in the N insulation to Earth. In addition, under these L/N reverse conditions, if there is a short-circuit between N and E then the N fuse (required in medical equipment) blows.

After manufacture or repair the mains parts of the instrument L/N can be accidentally reversed. Reverse polarity testing ensures the equipment earth leakage is acceptable under likely single fault conditions and safety tests may allow us to identify the source of the problem.

I have to agree with Sean; under SFC conditions where one supply conductor is O/C then if earth leakage increases then the E is completing a return path indicating poor seperation of L or N to earth, within the instrument, depending upon which conductor is O/C and polarity (of L/N). Under normal conditions N is close to E potential so degradation in N-E insulation would necessarily not be detected by testing under normal conditions would it? I will reason it out, later-on, below.

Another point is that safety testing should test for all combinations of SFC being considered; we are not just considering S/C but degradation over time up to the acceptable limits - systematic testing is a means to record values of combinations of the necessary tests laid down in the relevant standards for routine testing.

If the wiring in the mains supply (socket) is reversed i.e. L to N then is it not likely there will be a bang and the mains protection will kick-in? This is due, effectively, to the connection of L to E since N is close to E potential due to bonding of L-N at the power distribution source.

Are you using a medical electrical safety tester to BS EN 60601-1 Lindsay?

Let's assume, in this case, the N is effectively in parallel with an E (right back to the power distribution source) with an ammeter shunt resistance, R, in series with E in the safety tester (considering pure resistances only) and the S/C is instrument N to instrument E enclosure.

Total current = Ncurrent + Ecurrent. I suppose the majority of the load current will take the path of least resistance.....2/3rds of the current will flow down N: if N = R and E = 2R i.e. if the N and E are the same length and ammeter R is same as N and E path resistance (not unrealistic). Hence only 1/3rd of all S/C current flows in the E under these, albeit theroteical, conditions; therefore earth leakage would not equal load current by a long chalk so to speak.

Under these circumsatnces; for low power equipment (drawing 0.75A for example) then under normal mains polarity conditions but with N-E S/C Ncurrent would be 500mA and Ecurrent would be 250mA - lower than the maximum acceptable limit permitted. Normal testing only would not pick up the N-E fault. I would be interested to hear others comments on this.

PRO-Soft QA-90 Ver. 3.1.2
QA-90 Serial no. ;12169 04.03
Operator ;Whiston Hospital
Establishment ;Whiston Hospital
Date ;05/08/04 09:27:39

Equipment code ;Neutral Earth excess leakage simulation

Testgroup; Power Configuration; Limit; Value; Status;
Supply Voltage; L-N; ; 235.6; OK;
Supply Voltage; L-G; ; 237.0; OK;
Supply Voltage; N-G; ; 1.7; OK;
Current Consumption; ; ; 0.0; OK;
Protective Earth; ; 200.0; 76.0; OK;
Insulation Resistance; Mains - Case; ; 1.0; OK;
Earth Leakage Current; OS; 1000.0; 4.0; OK;
Earth Leakage Current; NC; 500.0; 4.0; OK;
Earth Leakage Current; OS-RM; 1000.0; 502.0; OK;
Earth Leakage Current; NC-RM; 500.0; 501.0; FAIL;
Enclosure Leakage Current; OS; 500.0; 0.0; OK;
Enclosure Leakage Current; NC; 100.0; 0.0; OK;
Enclosure Leakage Current; OE; 500.0; 4.0; OK;
Enclosure Leakage Current; OS-RM; 500.0; 0.0; OK;
Enclosure Leakage Current; NC-RM; 100.0; 0.0; OK;
Enclosure Leakage Current; OE-RM; 500.0; 502.0; FAIL;

***** UNIT FAILED TEST! *****

Remark
This is a demonstration of the importance of the reverse mains tests. A 470K resistor has been introduced between the earth and neutral of a test plug simulating a fault such as fluid ingress around a neutral fuse or breakdown of a suppresion capacitor. It often suprises people to see that the fault is only detected by the reverse mains tests. This is due to the fact that neutral and earth are effectively at the same potential.

Regards
Sean Fearon

Thanks for that Sean - it's pretty obvious really isn't it? - if you understand what the basics of testing are about.

We do the tests for a reason - lots of clever people have been there before us and established these tests ;-)

If we could manage with only one car on the road at a time it would certainly be safer. But that's not real life - just like using one piece of equipment at a time isn't achievable in theatres, ICU, HDU, CCU, etc. So the risk to a patient from a leaky transformer is that when the SFC is the loss of the earth, the excessive earth leakage could flow through some other piece of equipment (via a common earth connection) and then through the patient.

A large proportion of theatre equipment is mounted on stack systems or sat on top of each other (anaesthetic machine / ventilator/ monitor for instance) so they share a single earth connection. If you don't test for all possible sources of earth leakage you're leaving patient's and staff exposed to risk if a break in that earth occurs - because someone has wheeled the table over the flex - or simply tripped over it !

How much longer does it take to do all the tests compared to picking and choosing the tests ? What task is so vital that you need to save time on safety tests in order to do it ? ? Could you justify the alternative use of this saved time to a coroner ? ? ?

Some would probably like to justify using a PAT tester instead of a dedicated medical electrical safety tester for EST of medical equipment - so much cheaper.

To All

Thanks, much of the above shows how important it is to know and specify what we are talking about.
There seems to be some area of misunderstanding of each other and terms such as SFC. For exapmle we would all recognise a live - neutral reversal as a fault, but under test guidelines it is not considered to be a SFC (single Fault condition)!

If I have understood it correctly, Sean's example, as well as demonstrating excessive leakage in reverse polarity, also demonstrates an insulation resistance of 1Mohm! In itself worthy of a Fail!

That reverse polarity checks can show up a fault, where normal polarity does not, is not in dispute, and has been ably demonstrated. So far there have been 2 examples shown in answer to my first original question. No doubt there are some more, but not very many it seams.

As to the value, in terms of electrical safety, of these problems that have been found is perhaps another matter. However, I don't think that this forum is the place to answer that question.


This has been my venture into the EBME forum, and it has been an interesting - almost stressful experience!

Thanks to all

Lindsay

Quote:
"If I have understood it correctly, Sean's example, as well as demonstrating excessive leakage in reverse polarity, also demonstrates an insulation resistance of 1Mohm! In itself worthy of a Fail!"

Comment:
My understanding is that limits for insulation resistance are not specified in IEC 601-1.

Regards
Sean Fearon

I would just like to add my units, in mA, are incorrect - to illustrate with my hypothetical "low-power" instrument I should have used uA. Thanks for pointing that out Lindsay.

As you correctily say - reverse polarity is not classified as an SFC - therefore if L/N is reversed and there is a SFC e.g. degradation in insulation N-E, hence both conditions exist simultaneously, then there is a risk presented by N-E fault in the same way that there would be under normal conditions with a L-E fault I suppose. There would be an issue with this, would there not?