Arcing contact problems and voltage measurements - some thoughts

[alasdairp]

Hi folks
I am retired and only tend to read the weekly summaries. I read the long-winded thread re. the storage heater and MCB tripping (thread now closed) and wished to make a few different comments.

(a) measuring voltage on an arcing supply using a normal multimeter (even a true RMS Fluke) can be very misleading - the meter is trying to average (or compute the RMS value). Showing 210-230 volts may result from almost zero volts to 300 volts or even higher for the positive spikes if there are inductive loads around. So in this case you can't work out the power being dissipated in the 100 A main switch by simply multiplying an apparent "multi-meter read voltage drop" by the "average current".

(b) because the current is making and breaking (to some extent) you have a high frequency pulsing current of many amps through the MCB - which is not what they are designed to deal with. This will cause internal mechanical vibration and affect the tripping accuracy.

(c) someone excellently suggested a using a radio. A small (and cheap!) portable medium or long-wave AM radio is incredibly useful in hunting for arcing noise on supplies. When I was on a technical OFFER (the old Regulator) committee, I found that to be the best way to initially check for arcing on rural supplies.

(d) Finally, do not forget to take into account the incoming supply voltage. All appliances (including storage heaters) are now designed for 230 volts RMS that was agreed back in the 1990s. However, the UK ESI, unlike most of Europe, chose not to implement the 240 V to 230 V change - instead changing the +/- voltage tolerances to -6% and +10% (216 to 253 volts). I have found many places, especially close to rural renewable generation feed-in substations, where it regularly hovers around the 250 V mark (and sometime above 253 V!). This is especially the case near to straw burners, waste incinerators and windfarms who generate at night and who are allowed to increase their supply voltage (by +10% over nominal, I believe) in order to deliver their agreed power to the network - that needs to be better regulated. A large 3.5 kW storage heater will draw 15.2 A at 230 volts - a resistive load of c. 15 ohms - so increasing the supply to 250 V RMS will increase the current to c. 16.7 A (actually it would be a little less as the increased temperature of the heating element would increase the resistance slightly).

Just food for thought (and probably, responses and challenges!)

 

[static zap]

( a) is interesting to note )
I do like c)
But feared being shot down spook hunting !
( Human managed arc fault detection )
--Must discus hum from a Kettle element some time ---

 

[buzzlightyear]

good read, thanks for that .he's the Stephen hawking's of the electrical world .

 

[marconi]

These two statements were made:

1) My mother in law heats her house by 4 Night Storage Heaters,

2) Voltage did not fluctuate until 4th new storage heater was energised the new one (dint know why this is a mistery to me as much as you)

It is not clear from 2) whether NSHs 1 to 3 were energised and then the new 4th one, or only the new 4th one was energised.

I am curious if the arcing commenced/voltage fluctuated only after NSHs 1 to 3 were already on and then NSH 4 was turned on. In this case some threshold has been exceeded or condition met or both. The presence of arcing/voltage fluctuation seems to me too binary.

Thoughts?

 

[static zap]

.... In this case some threshold has been exceeded or condition met or both.....

Thoughts?

Weakness / limiting factor in supply chain ..time will tell .

 

[The Ghost]

Hi @alasdairp and welcome. I looked at the web site you referenced in your profile. I have a passing interest in wi-fi in schools and possible outcomes and ramifications of wi-fi and children, and maybe even staff. I was watching a you tube offering on the subject which the presenter suggested that wi-fi has an effect on the DNA of children. He went into some depth on this subject suggesting cellular degradation of the eggs in girls and similar types of degradation in boys. What is your view on such an assertion?

 

[KEV 1 N]

In regards to what Vortigern mentioned. I have often thought about all these “signal waves” going through the air and are they actually dangerous. The use of wi fi is more widespread now.....it’s every where. Mobile phones are everywhere also and being used constantly, more than ever with Snapchat, Twitter, Facebook etc.
I remember working on a roof testing some air con unit supplies and the there was a mobile phone mast on the roof, now I kept away from it but after about half an hour I could feel a slight headache starting, so it makes you think about the effects these may be having.

 

[Strima]

I remember working on a roof testing some air con unit supplies and the there was a mobile phone mast on the roof, now I kept away from it but after about half an hour I could feel a slight headache starting, so it makes you think about the effects these may be having.

As soon as you leave the area the effects of non-ionising radiation stop and you should quickly recover with no long term effects. Symptoms are cold and flu like symptoms and in extreme cases heating of the skin.

 

[alasdairp]

These two statements were made:

1) My mother in law heats her house by 4 Night Storage Heaters,

2) Voltage did not fluctuate until 4th new storage heater was energised the new one (dint know why this is a mistery to me as much as you)

It is not clear from 2) whether NSHs 1 to 3 were energised and then the new 4th one, or only the new 4th one was energised.

I am curious if the arcing commenced/voltage fluctuated only after NSHs 1 to 3 were already on and then NSH 4 was turned on. In this case some threshold has been exceeded or condition met or both. The presence of arcing/voltage fluctuation seems to me too binary.

Thoughts?

The whole saga was rather odd. I suppose the quite old 100A main switch could have been damaged if it were switched on-off when all 4 heaters were fairly cool but online waiting for a charge-up. The instantaneous current would be high and possibly could do some initial damage to the quite old Main Switch contacts? Something recent must have made the MS to start to fail. I know that MS are rated to switch very high currents, but their main purpose is to just serve as an isolator rather than to actually switch high loads. The fact that the new heater (and cable) did not trip an MCB on the main (not off-peak) board might be because only the one heater was drawing current. All 4 heaters coming on all together from cool would present a high switching load that could maybe start the now failing Main Switch arcing and that was the final straw for the problem MCB (which, by the sound of it, should have been 20 A rated). We will never know.

 

[alasdairp]

As soon as you leave the area the effects of non-ionising radiation stop and you should quickly recover with no long term effects. Symptoms are cold and flu like symptoms and in extreme cases heating of the skin.

We will have to agree to disagree. What you write is the conventional view ("if it doesn't electrocute you (electricity) or heat you (RF) then it won't hurt you."). It was one I grew up with and qualified under in the 1970s. I am now a Trustee of a large cancer charity (Children with Cancer UK) and I spend a lot of time reviewing the published scientific literature on environmental causes of cancer and ill-health. Ionising (nuclear) radiation only causes about 0.5% of all cancers - the rest are caused by all sorts of environmental and lifestyle exposures and by things that do not directly break covalent bonds. EMFs (ELF and RF) have been repeatedly shown to promote (if not directly cause) adverse effects in living animals and people. Our RF exposure has changed enormously. I attach a graph I produced for a conference in 2012. It shows a 1000000000000-fold increase in our microwave RF (mobile phones and WiFi) since the 1970s. The "Bioinitiative Report" is a comprehensive review of the science.

 

[Lucien Nunes]

Some good points there Alasdair. The likely inaccuracy of measurement of non-sinusoidal voltage should have been raised at the time, but I think we got bogged down trying to work out whether the drop referred to was in the switch or the supply cable. In any case we never even found out what instrument took the measurements. It's still not clear to me how any significant drop could have occurred in the switch as the mean dissipation could not have been more than a few dozen watts without visible signs of overheating. The damage within the case loooked more like the result of brief, intense arcing, such as when closed onto a short.

To put this to bed, I would like to do a practical test of the sensitivity of an MCB to extreme waveforms, e.g. by playing an arbitrary waveform generator through a laboratory amplifier and transforming down to get a test supply of up to say 100A pk. My hunch is that the MCB will stay in spec for thermal tripping accuracy wrt. RMS current, but the magnetic trip could do anything. I might try this if the Christmas celebrations start to drag on too long. However, since there was a resistive load (we think) in series at all times, we were generally in agreement that the magnetic trip was never involved.

The bottom line is that although we know how the fault was cured, we don't truly know the nature of the fault itself.

 

[Bul]

I think that this has been the most interesting post of the year !

 

[ipf]

The bottom line is that although we know how the fault was cured, we don't truly know the nature of the fault itself.

I think that sums things up pretty well, Lucien.

 

[static zap]

TESTS.... of the sensitivity of an MCB to extreme waveforms, e.g. by playing an arbitrary waveform generator through a laboratory amplifier and transforming down to get a test supply of up to say 100A pk. ...The magnetic trip could do anything. I might try this if the Christmas celebrations start to drag on too long..

Have wondered if arc may have fluctuating DC componenets that may be occasianally reversing . If the intermitency - hits natural resonance
with occasional impulses .(Sods law and when you want random symetry ... a DC componenet may desensitize ... magnetize ..then reverse (expecting resonances to change with manufacturer geometry)
-- Keep up the up the LAB work Lucien --

....... in extreme cases heating of the skin.

We were warned eye corneas have little blood flow to remove heat ..
(remembered watching my dinner in the microwave -- cyclic pattern)

 

[Midwest]

I think that this has been the most interesting post of the year !

It would be, if I knew what the hell they are banging on about. I reckon they are just taking the ----, and saying anything that comes into their heads.