Will that fuse (or mcb) blow or how soon will it blow if I overload the current by 10-20%?

Forums › Home Workshop DIY › Will that fuse (or mcb) blow or how soon will it blow if I overload the current by 10-20%?

Huchiz

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#189090 16-Dec-2015 16:51

I have one circuit which has two powerpoints on it, one for the dishwasher and one for the coffee machine/blender.

Dishwasher has peak power of 2200W, so 9.2A. It runs for long time every night, sure it doesn't always heating up but who knows when.
Coffee machine is 1700W, 7.1A, but I assume the thermal coil doesn't work for a long continues time, may be a couple of minutes.
Blender is 2000W, 8.3A. But it only runs for less than 60 seconds. And I won't use it together with the coffee machine.

So it could be either 9.2A+7.1A=16.3A for a few minutes or 9.2A+8.3A=17.5A for under a minute.
The old style fuse for this circuit is 15A rated, so that is a 9-17% overload. Will it blow? Or how soon?
If I upgrade the fuse with the retro fit "plug in" mcb, HPM recommends 16A mcb for a 15A fuse, will it cut power even more easily?

richms

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#1451733 16-Dec-2015 17:03

It should be fine. Have had 2 2400 watt heaters (really about 2100 on 230v) on a 15A for all day every day at a previous work in winter.

Richard rich.ms

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#1451735 16-Dec-2015 17:06

This I am sure. I think. ...
Just because something is rated at 15A does not mean it won't have ANY problems at 14.99 ... or 14.8 ... or 14.4 ...
Just because something is rated at 15A it does not mean it WILL blow at 15.01 ... or 15.1 ... or 15.2 ...

But as you suggest you are a bit beyond that if they all hit peak together. So the gamble is yours. If it were me ... if there is no other way, don't run them all at the same time!

Huchiz

221 posts

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#1451738 16-Dec-2015 17:17

joker97: This I am sure. I think. ...
Just because something is rated at 15A does not mean it won't have ANY problems at 14.99 ... or 14.8 ... or 14.4 ...
Just because something is rated at 15A it does not mean it WILL blow at 15.01 ... or 15.1 ... or 15.2 ...

But as you suggest you are a bit beyond that if they all hit peak together. So the gamble is yours. If it were me ... if there is no other way, don't run them all at the same time!

I know. The problem is you can't really avoid the dishwasher as it runs for 2-3 hours, though peak power (heat element) duration should be very short, but it's hard to predict when. :D

Huchiz

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#1451740 16-Dec-2015 17:19

richms: It should be fine. Have had 2 2400 watt heaters (really about 2100 on 230v) on a 15A for all day every day at a previous work in winter.

Really? So in that case, it should be no problem at all. Could I ask was it a fuse or MCB? :D

richms

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#1451741 16-Dec-2015 17:20

Breakers have a time to trip as part of their design. Its to do with the curve they select, there is a few types.

They are made to protect wiring, which will overheat on a sustained overcurrent. Short duration isnt a problem and breakers are designed to not trip on that as its not a hazard for the wires.

Richard rich.ms

JeremyNzl

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#1451745 16-Dec-2015 17:23

All reputable MCB brands will have there power curve in there specs, These will be somewhere online.
depending on the tempreture most will stand a constant 10-20% for a period of time.
MCB's are there to protect the cable that runs from them , upgrading MCB's without checking the cable or upgrading it could cause a fire worst case.
Heavy inrush current near peakloading will trip the breaker.

richms

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#1451746 16-Dec-2015 17:24

Chart for a random schnider C curve I found says 3600 seconds for < 1.3x so you should be Good. Its been a long time since I looked into anything like that tho.

There is inrush to worry about with the motors tho, but if it does trip then worry about upgrading the circuit.

Richard rich.ms

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Huchiz

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#1451756 16-Dec-2015 17:37

richms: Breakers have a time to trip as part of their design. Its to do with the curve they select, there is a few types.

They are made to protect wiring, which will overheat on a sustained overcurrent. Short duration isnt a problem and breakers are designed to not trip on that as its not a hazard for the wires.

What is the usual wire gauge they used on normal powerpoint circuit?
My house is late 70's and still has the old style fuse box. I opened some of the powerpoints and switches, they are PDL 600 series, not too old, and the cable inside looks quite new. But my eyes are not good enough to tell their gauge.

timmmay

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#1451757 16-Dec-2015 17:39

Run your dishwasher overnight on its timer. We do it all the time as we have flick electric and power is half the price overnight, sometimes less.

gregmcc

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#1451763 16-Dec-2015 17:43

Huchiz: I have one circuit which has two powerpoints on it, one for the dishwasher and one for the coffee machine/blender.

Dishwasher has peak power of 2200W, so 9.2A. It runs for long time every night, sure it doesn't always heating up but who knows when.
Coffee machine is 1700W, 7.1A, but I assume the thermal coil doesn't work for a long continues time, may be a couple of minutes.
Blender is 2000W, 8.3A. But it only runs for less than 60 seconds. And I won't use it together with the coffee machine.

So it could be either 9.2A+7.1A=16.3A for a few minutes or 9.2A+8.3A=17.5A for under a minute.
The old style fuse for this circuit is 15A rated, so that is a 9-17% overload. Will it blow? Or how soon?
If I upgrade the fuse with the retro fit "plug in" mcb, HPM recommends 16A mcb for a 15A fuse, will it cut power even more easily?

there is a lot of depends...... Download the data sheet for the proposed HPM circuit breaker your are thinking of using, there will be a curve graph showing time vs. current

richms

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#1451764 16-Dec-2015 17:44

Huchiz:
richms: Breakers have a time to trip as part of their design. Its to do with the curve they select, there is a few types.

They are made to protect wiring, which will overheat on a sustained overcurrent. Short duration isnt a problem and breakers are designed to not trip on that as its not a hazard for the wires.

What is the usual wire gauge they used on normal powerpoint circuit?
My house is late 70's and still has the old style fuse box. I opened some of the powerpoints and switches, they are PDL 600 series, not too old, and the cable inside looks quite new. But my eyes are not good enough to tell their gauge.

Nowdays 2.5mm or 4mm depending on length. Old days 1.5 was common, before that in prehistoric times they measured them differently with a / in the middle and I have no idea.

There are tables in the standards about what current they are good for under what conditions. They keep amending the standards which means the little I learned 18 years ago is meaninless now.

Richard rich.ms

chopsuwe

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#1452373 17-Dec-2015 16:02

It's been a while since I studied this but going by the curves on this web site, you should be ok for around 10 minutes.

In short the breaker is designed to allow for short overloads and will break more quickly the higher the current.

http://www.studyelectrical.com/2014/07/miniature-circuit-breakers-mcb-types-characteristic-curves.html?m=0