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Posted By Topic: Volt drop damaging electronics

Mar 05 2020 08:40

Hi there. I have a rural customer with the following problem. Every time they use a table saw (2200W) there is a significant volt drop causing problems for electronics in the office e.g the printer restarts and is potentially the reason for a few hard disks being fried and other issues with desktop computers not working. Supply voltage at main switch is 235V and i've noted a drop down to 210V when the table saw kicks in. They do have a spare phase available which is my first though of a solution (i can shift the shed feed to this spare phase) however i'm wondering if there should be any other considerations because while this may solve the issues in the office it will also just shift the issue to the other phase and future equipment that may be used in the shed. TIA

Mar 05 2020 09:35

The cheapest solution for the customer might be to buy a small UPS (un-interrupted power supply) for the computer equipment. Start around $150. It gives battery-backed supply for a few minutes and smooths all the lumps out of a wobbly supply.

Mar 05 2020 10:56

To elaborate on what SaintAlan has said, an online UPS - these clean/filter the supply more effectively than the cheaper offline type UPS's.

Mar 05 2020 11:18

Surges and spikes, rather than brownouts, are likely cause of failed electronics. And where you have dips in an inductive system you'll also have peaks.

The vast majority of modern single phase switchmodes will happily handle spec'd voltages 90-250 Vac. Though that isn't a guarantee for all. So excessive dips from 230 V aren't even out of the ordinary operating range for them. They will all have a decent leeway for surges above 250 V but once that is exceeded, failure is swift.

UPS's can generally also be tossed into that group of doesn't handle surges well. Although it could have higher max voltage this is not in the typical UPS spec's. It's still the same switchmode structure as the power pack it's protecting. So I wouldn't count on a UPS as reliable fix unless you are planing on replacing it a lot.

Not being an electrician, I'm not familiar with appropriate supply fixes.


Mar 05 2020 11:28



Mar 05 2020 12:01

235 V is within permitted range [ESR 28]

Prolonged dropping to 210 V on load means the load is greater than permitted for the cable over the distance. The max permitted VD is 5% (11.5 V); ie to 218.5 V.

However it seems this is during start-up, when you can expect up to 6 x FLC for DOL stating. That issue could perhaps be dealt with by using non-DOL starting; but you might still have a problem if the saw ever gets fully loaded.

You could try loading it up and measuring, but easier to:
- read the FLC from the tally-plate;
- check the size of the cable (each section, mains, submains, final subcircuit);
- measure the cable route distance.
Then do a VD calculation.

Chances are either it was never installed to cope with that sort of load; or other load in the rest of the installation has increased.

It may also be that the network can't actually supply the load capacity the customer is paying for.

The only effective way to deal with prolonged volt drop is to upgrade the supply; so as to stay within the permitted limits.


Mar 05 2020 15:37

If buying a computer make sure you get one able to cover the startup loading of the computer system running from the UPS.

You can't simply use the nameplate of the computer equipment = the UPS maximum output amps, this does NOT cover the actual VA required, a switchmode power supply (SWPS)has a startup surge of up to 6 to 10 timer x running load and is very spike of current required.

Good SWPS are expensive and most computer manufacturers don't use them due to the extra cost, unfortunately, it means a larger power supply peak capacity is required and this means a extra cost in the UPS.

Mar 05 2020 16:14

I may opt for a retrofit soft starter option. There's not much i can save on VD by changing cables. From main switch to final sub circuit it would be no more than 35m with 30 of that being 16mm already. They have already purchased some cheap UPS machines however are already having battery problems with those so it would be worth getting some better ones.

Mar 05 2020 16:49

No matter how expensive the UPS is, if it uses lead-acid batteries then you're up for regular battery replacements, every five years or so.

Lithium's are finally appearing for UPS's. I can't say what the expected life span will be for those but my guess is they're probably worth paying extra for.


Mar 05 2020 19:49

Get the lines co to check all the connections between POS and the transformer, seen this several times over the years where a join degrades and causes the issue your describing, sometimes the lines co contractor needs additional encouragement as they dont check all the connections - just the two ends.

Mar 05 2020 20:54

Have you done a loop impedance test at the main switch? Or checked for voltage rise on the neutral under load?

Mar 06 2020 19:50

Remember that the lines company is also allowed to supply you up to 6% less than 230V, and volt drop has to further be taken off this.


Mar 06 2020 19:56

True; but from a compliance perspective we just work off nominal voltage.

Mar 07 2020 13:28

My point is more that if the lines company is supplying you 230V -5% at the point of supply, and you're then losing another 4% within the property, you can be seeing 210V and everything is perfectly legal.

I believe the regulation also excludes short-term and momentary excursions caused by e.g. motor starting.

Mar 07 2020 13:57

My point would be that even if there is 6x starting current as Alec suggested, that would be around 60A and within a nominal supply capacity. Dropping from 235V to 210V is more than 10%. PaleAle said that this was measured at the main switch so it is way over what is allowed.

I would suspect that there is something wrong with the supply.

Mar 10 2020 09:55

Hi, I agree with a good check of the incoming supply. Make sure you check all the N connections in particular as these can get missed. I have even run into a degraded connection coming of the N/S of a cable from when it was ringed with a knife.

Mar 10 2020 12:03

Maybe there is a bad joint in the supply line and it would be good to check. However I have done a lot of work in construction site huts at the end of temporary lines or on portable generators. Both the voltage and frequency can be quite lumpy.
I found a UPS is wonderful protection and am using one now in my office in Auckland, overkill maybe.