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Topic Title: How to verify electronic supply distribution
Topic Summary: Equipment required, testing methods and assumptions
Created On: 21 June 2013 08:52 AM
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 21 June 2013 08:52 AM
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I have a situation whereby a factory distribution is provided by an invertor supply providing 115Vac @ 400Hz. The installtion will need to be tested before we can use it.

My questions relate to how I would show compliance with BS7671 for the installation and how this reflects upon the testing methods. This would apply not only to this installation, but to electronic supply sources supplies throughout the UK.

1. As the supply is 400Hz I cannot find any test equipment to provide the Zs and pfc at this frequency. Most is rated at 50Hz. Does anyone know of test equipment I can use to cover this frequency range?

2. As the supply is via an electronic output, will a measured Ze value be of any use?

3. How does/would transformer Ze differ from electronic Ze?

4. Would Zs & pfc measurments be of any use given the output would be protected by fast fuses, automatic rolloff, or by blowing open circuit the device itself?

5. I was thinking of using (R1+R2) measurements and applying a theoretical value of Ze and pfc, but as the frequency is higher, would this still be applicable? Does anyone know of a formula I could use?

6. The installation will have and 400Hz rated RCD. Would a 50Hz tester be ok for this test, or would I need a 400Hz tester given the leakage current will affected by frequency.

Many thanks in advance for any help you may be able to provide.
 21 June 2013 03:37 PM
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What does the invertor supply - motors, induction heaters ? Is it a stand alone system or retro fitted together ?

Empty barrels make the most noise.
 21 June 2013 05:22 PM
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I would not expect the Ze to be much different to its 50Hz measurement. You cannot use a normal 50 Hz loop tester, but you can make DC measurements and unless the runs are long will be adequate to estimate the PFC of each circuit. The overall system PFC will be similar to the inverter maximum output, the electronics will protect themselves at little more than the maximum current to prevent damage. A transformer will supply a short circuit current which is much higher than the maximum load, electronic supplies will not. Your RCD tester may be OK, but the accuracy may be reduced. The test load imbalance is introduced by a resistor which is not frequency sensitive.

What is the inverter output power?

 23 June 2013 09:54 PM
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I guess my first question would be - what's the intended method of protection against electric shock? There are many possibilities - separated system, double insulation etc as well as ADS. If it's not ADS then you might not be required to know/control Zs at all.

I've only come across 400Hz systems in references in books - am I right in thinking that 400Hz is chosen as it's considered safer than the same voltage at 50Hz? If so, and if the system is ADS, - RLVS at that - then presumably the required disconnection times could be longer too?

- Andy.
 28 June 2013 11:13 AM
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I agree with your comments, but how do I confirm the theory? BS7671 requires expectations based upon supported test evidence.
Do you have an idea as to what the RCD test error is likely to be?
It appears we agree 50Hz equipment is not suitable, but does anyone know where I can get hold of 400Hz rated equipment? I cannot be alone in this requirement.

The protection against electric shock would primarily be RCD. As the equipment does not comply with BS7671 there are some exposed mains potential points that could lead to unintentional contact.
Secondary protection would be ADOS. This would need to be verified as the users are not trained or skilled and at times unsupervised. The supply is rated at 50kVA.
400Hz is used, as the rotary equipment is a lot smaller, hence weight is reduced. It still bites when angered!
 28 June 2013 11:27 AM
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Presumably the manufactured product would need to be I&T when in service, so how do the aircraft guys manage?


 28 June 2013 11:48 AM
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John Peckham

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I have a Megger LT300 loop tester that is suitable for 400Hz supplies. I purchased it to use on some kit in a container that was heading for a hot and dusty place.

Talk to Megger techinical as the Megger Lop/RCD tester has a frequency measurement range of 25-450Hz.

My other Megger LTW loop will refuse to do the test if the frequency is outside the range of 49-51 Hz.

John Peckham

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