Measuring Inrush Current

[Davisonp]

I am trying to find the maximum current drawn through a switchfuse circuit supply circuit (60Amp switchfuse). I have been told that there should be a large inrush current when some equipment kicks in, but I would like to confirm the actual inrush current.

I have had a look at cheap current data loggers, but from the instructions, I do not think they could show me when a current surge happened and the maximum current drawn.

Does anyone know of a cheap energy (or current) loggers that would help or have any ideas experience they can pass on?

Thanks

 

[The Ghost]

Normally you can get the inrush currents (graph) from the manufacturer of the equipment no?

 

[hightower]

An ammeter with a 'hold' function, should be a pretty standard feature on most clamp meters. It's on mine and it cost no more than £25.

 

[Davisonp]

Normally you can get the inrush currents (graph) from the manufacturer of the equipment no?

I am asking the client for any documentation supplied by the installing electrician, but from the work I have seen, I would not be surprised if there is none.

 

[Davisonp]

An ammeter with a 'hold' function, should be a pretty standard feature on most clamp meters. It's on mine and it cost no more than £25.

The equipment is swimming pool equipment, and I am not sure if it can be triggered. I think it is just running constantly, then kicks in when needed, like a fridge motor. I have no idea how long I will need to wait to get a reading, or if there is more than one piece of equipment that will create an inrush. I was hoping to record the current over 24 hours and get a max current from that, but all clamp meters I can find auto turn off after a 20 minutes.

 

[Davisonp]

An ammeter with a 'hold' function, should be a pretty standard feature on most clamp meters. It's on mine and it cost no more than £25.

I may also be jumping the gun a bit. It may be that when I start investigating in earnest, all becomes clear without 24 hour load testing. Just thinking and trying to get my head round the problem and get a plan to find a reasonably accurate max current on this switchfuse.

 

[PEG]

Hi,if you have several pieces of equipment which are liable to cause this phenomenon,and they individually switch when required,you may have to consider more than one piece switching on at a time,and cater for worst case scenario.

 

[Marvo]

Inrush current isn't easy to measure, you need an instrument with high sample rate and wave capture abilities. Data loggers are usually low sample rate, even ammeters with a max hold facility won't have a sample rate high enough for accurate inrush measurement.

Usually you wouldn't need to ascertain precise inrush current values, manufacturers values or rule of thumb values would generally be fine to design the supply circuit. If it's an existing circuit and you suspect inrush current is causing a tripping problem then run circuit impedance tests and see if you can replace the OCPD with a C or a D curve. Sometimes you might also consider using a short delay control timer on one of the loads if there's multiple motors on a single circuit that are likely to start together when power is applied.

 

[Davisonp]

Hi,if you have several pieces of equipment which are liable to cause this phenomenon,and they individually switch when required,you may have to consider more than one piece switching on at a time,and cater for worst case scenario.

Yes, I plan to keep this in mind.

 

[Davisonp]

Inrush current isn't easy to measure, you need an instrument with high sample rate and wave capture abilities. Data loggers are usually low sample rate, even ammeters with a max hold facility won't have a sample rate high enough for accurate inrush measurement.

Usually you wouldn't need to ascertain precise inrush current values, manufacturers values or rule of thumb values would generally be fine to design the supply circuit. If it's an existing circuit and you suspect inrush current is causing a tripping problem then run circuit impedance tests and see if you can replace the OCPD with a C or a D curve. Sometimes you might also consider using a short delay control timer on one of the loads if there's multiple motors on a single circuit that are likely to start together when power is applied.

Thank you.

It is not a tripping problem, in fact, almost the opposite. Someone installed these pool items, but there are a couple of things about the installation that ring alarm bells with me. The installer told the client to upgrade a cable from the main house fuse from 60 Amps to 100 amps or the inrush current from the pool equipment would blow the 60 Amp fuse. That was over a year ago and the fuse has never blown, so before the expense of upgrading the cable and switchfuse, I want to find out the inrush current and the maximum current to see if there really is a problem.

 

[Marvo]

Usually you'd only upgrade the cable if the voltdrop during start-up was excessive or if there's an overload issue. What exactly is running ie make and model number of the pumps and list any other equipment such as chlorinators, heaters etc.? What type are the fuses? How come you're investigating this system if there hasn't been any problems?

 

[PEG]

Yep,requires more investigation,of what else has been done in error,because that increase in fuse value,is not a way round that problem.

 

[Marvo]

Hardly headline news..... but I'm a bit confused.

 

[Davisonp]

Yep,requires more investigation,of what else has been done in error,because that increase in fuse value,is not a way round that problem.

1) The installer has not checked the household fuse. It could be 60 or 80 Amps according to the fuse carrier. The installer should not have installed equipment that can blow a 60 amp fuse without checking this first.

2) The 60Amp switch fuse was originally used just to supply the garage CU. It looks like this installer has removed the 10mm SWA from the garage fuseboard and inserted the 10mm live and neutral cables unprotected into a harting connector and the earth core of the SWA into an MET . The SWA should be terminated through a gland into an enclosure then through conduit or something, but the installer has removed the SWA gland taking the SWA into the old consumer unit and leaving the SWA steel armour loose. Then he has taken 25 mm tails from the harting connector back into the old CU, but they will not fit through the old SWA hole, so he has knocked out a second 20mm knockout so now the live goes through one hole and the neutral through the second hole, no grommets or glands on these cable entries. I have not had a look at the installation at the other end of the cable yet.

 

[Davisonp]

Usually you'd only upgrade the cable if the voltdrop during start-up was excessive or if there's an overload issue. What exactly is running ie make and model number of the pumps and list any other equipment such as chlorinators, heaters etc.? What type are the fuses? How come you're investigating this system if there hasn't been any problems?

Yes, it happens a lot, questions asked, then folk say “Why” and the back story starts coming out in bits and pieces, out of context and out of sequence

I was contacted by a client who asked me to quote for changing a garage fuseboard, their main house fuseboard, and a cable and fuse that feeds the garage fuseboard as they had new equipment installed for their swimming pool and the installer told them the 60Amp fuse and 10m Cable supplying the garage fuseboard was inadequate and they needed to upgrade the this before the fuse blew.

My first look was late in the day on the way home, so in the first look all I had access to was the garage, the household supply with main consumer unit and switchfuse.

I asked the client if the 60Amp fuse had blown and they said it had not and it was between 1 year and 1 and a half years since the equipment was installed and they were only just getting round to the upgrade.

So, I question if the upgrade to the fuse and cable is necessary and am trying to work out a way to confirm if the current required, especially since the installer emphasised the inrush current and it’s immanent threat to the 60Amp fuse.

 

[ipf]

Yes, it happens a lot, questions asked, then folk say “Why” and the back story starts coming out in bits and pieces, out of context and out of sequence

I was contacted by a client who asked me to quote for changing a garage fuseboard, their main house fuseboard, and a cable and fuse that feeds the garage fuseboard as they had new equipment installed for their swimming pool and the installer told them the 60Amp fuse and 10m Cable supplying the garage fuseboard was inadequate and they needed to upgrade the this before the fuse blew.







It's been installed in the last 18 months, so contact this installer for some load details of their installation. Get them to explain the 'inrush current' and the resultant load requirements. If it is inadequate, as they say, why did they install and connect the equipment?
Were any completion certificates issued?

 

[ipf]

Yes, it happens a lot, questions asked, then folk say “Why” and the back story starts coming out in bits and pieces, out of context and out of sequence

I was contacted by a client who asked me to quote for changing a garage fuseboard, their main house fuseboard, and a cable and fuse that feeds the garage fuseboard as they had new equipment installed for their swimming pool and the installer told them the 60Amp fuse and 10m Cable supplying the garage fuseboard was inadequate and they needed to upgrade the this before the fuse blew.

My first look was late in the day on the way home, so in the first look all I had access to was the garage, the household supply with main consumer unit and switchfuse.

I asked the client if the 60Amp fuse had blown and they said it had not and it was between 1 year and 1 and a half years since the equipment was installed and they were only just getting round to the upgrade.

So, I question if the upgrade to the fuse and cable is necessary and am trying to work out a way to confirm if the current required, especially since the installer emphasised the inrush current and it’s immanent threat to the 60Amp fuse.

It's been installed in the last 18 months, so contact this installer for some load details of their installation. Get them to explain the 'inrush current' and the resultant load requirements. If it is inadequate, as they say, why did they install and connect the equipment?
Were any completion certificates issued?[/QUOTE]

 

[Davisonp]

It's been installed in the last 18 months, so contact this installer for some load details of their installation. Get them to explain the 'inrush current' and the resultant load requirements. If it is inadequate, as they say, why did they install and connect the equipment?
Were any completion certificates issued?

[/QUOTE]

Thank you.

I have asked for any documentation, but never considered contacting the installer. That is a good point.

 

[netblindpaul]

If you've a spare £3-5k ish buy a power analyser, stick it on there for a week or a month and you'll have your answers once you have analysed the data provided.
Done it a few times with my kit and it works well if you set it up right.

 

[Darkwood]

I would play the old game here, if no issue exists then don't worry until it does, many companies, manufacturers etc play on a worse case scenerio and this is brainwashed into the installers who usually have no knowledge of the actual calculations behind it, it's called protecting there arse in a market they are probably importing foreign equipment to, I see it all the time with chinese imports like tranformers or motor suggested fusing when in fact they are way of mark and would cost my customers thousands more to take their advice.

A respectable company will design a panel to minimise inrush ie.... by staggering inductive load inrush when demanded if multiple units are called at the same time, I cannot say the system you have there is like this but given no-one has had any issues then maybe you need to discuss this with the person that raised this concern in the first place although given your asking the question then I'm not sure you could challenge them and take the upper hand if indeed they are wrong, having said that there is no arm in asking them as a direct question as oppose to a challenge of design.

 

[Wilko]

Hi - Just my opinion, but after 18mths use and possibly 1000s of pump power cycles I think its proven the service fuse is ok with the inrush current. Keep in mind the BS88 63A fuse will tolerate more than 200A for 10sec without melting and the 40A B MCB in the final circuit (say) will trip instantly. Whether the rest of the design is ok (possible max demand issue?) and there are the installation defects to chew through.