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Ewi

Ewi

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Hey all.

Im new to this forum. Seeing how limited d web is to rectifying any electrical queiries, i thought i might find this useful.

I am a warranted electrical domestic licencee. (Based on IET) and possess a Higher Diploma in Electrical Engineering. I live in Malta (EU) which uses a UK electrical based infrastructure. (TT system).

Just yeaterday as i was looking into a fault at a client, i realised something very strange regarding current draw. I plugged a 0.9KW hairdryer (3.9A @ 230v) into wall socket outlet and clamped live and neutral seperately. L read 4.2A and N read 3.6A. Unless my clampmeter requires calibration what is the reason of this variation???

The worst part is that when i clamped the hot conductor at the consumer unit main DP MCB, it marked that it was drawing 9.2A!!! (Yes, I have deducted the current that was being drawn from the house at that time). That is more than double what it should be marking. I understand that the cables from socket outlet to consumer unit might carry some extra resistance hence draw more current, but surely not to that extent!!!

Can anyone explain this other discrepancy???
 
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Re #1: You might want to re-consider what you wrote:

I understand that the cables from socket outlet to consumer unit might carry some extra resistance hence draw more current, but surely not to that extent!!!

What is the accuracy of your ampclamp?

[ElectriciansForums.net] Current draw discrepancy
 
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Ewi said:
Hey all.

Im new to this forum. Seeing how limited d web is to rectifying any electrical queiries, i thought i might find this useful.

I am a warranted electrical domestic licencee. (Based on IET) and possess a Higher Diploma in Electrical Engineering. I live in Malta (EU) which uses a UK electrical based infrastructure. (TT system).

Just yeaterday as i was looking into a fault at a client, i realised something very strange regarding current draw. I plugged a 0.9KW hairdryer (3.9A @ 230v) into wall socket outlet and clamped live and neutral seperately. L read 4.2A and N read 3.6A. Unless my clampmeter requires calibration what is the reason of this variation???

The worst part is that when i clamped the hot conductor at the consumer unit main DP MCB, it marked that it was drawing 9.2A!!! (Yes, I have deducted the current that was being drawn from the house at that time). That is more than double what it should be marking. I understand that the cables from socket outlet to consumer unit might carry some extra resistance hence draw more current, but surely not to that extent!!!

Can anyone explain this other discrepancy???
Click to expand...
Something else plugged in on that circuit? how did you measure the L and N of the Hair drier?
 
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The live and neutral current must be equal, so clearly the meter was giving incorrect indications.Either it is faulty, or it cannot handle the current waveform that the hairdryer produces. Some hairdryers half-wave rectify the current to the heating element on low setting, i.e. although the supply is AC the current drawn is pulsating DC. This can cause an AC-only meter to read incorrectly and inconsistently. Even without the overall half-wave rectification, the waveform can be noisy and distorted due to the DC fan motor which also has a rectifier.

You cannot add or subtract currents from the rest of the installation to calculate the expected total reading at the consumer unit, unless they are all in-phase. If there is significant waveform distortion in the hairdryer current, then it will not add algebraically to the current from a true resistive load, for example. Therefore your expected total current may have been in error.
 
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Thanks for your replies. However i still find myself confused.

Maybe i need to clarify..

The current reading at the consumer unit which is directly connected to the kwh meter is the actual current draw. The consumer pays depending on the current draw at the kwh meter.

In this case, with the hair dryer turned off, the kwh meter read 2.1A. With the clampmeter left in same place, when i turned the hairdryer on it read 11.3A.(which means that the hairdryer was actually drawing 9.2A!! I.e the consumer will pay on the 9.2A and not on the 3.9A rating of device). When clamped at the socket outlet directly where the hairdryer was connected, it read an average of 3.9A which makes sense according to its rating (900W at 230v.). My question is where is this massive current discrepancy coming from?

(Maybe i need to scrap my clampmeter??)

Thanks again
 
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Hi - I don't know what's really going on, but I suspect it's a combination of your meter and how you are using it. For interest you could make up an extension cord and test some loads. Type of load, position of clamp and proximity to other fields may all affect the reading.
 
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Please clarify (-even more :) )the it I have highlighted in bold - is it the kWh meter or your ampclamp (or the hairdryer).

In this case, with the hair dryer turned off, the kwh meter read 2.1A. With the clampmeter left in same place, when i turned the hairdryer on it read 11.3A.

and later:

When clamped at the socket outlet directly where the hairdryer was connected, it read an average of 3.9A which makes sense according to its rating (900W at 230v.).
 
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