Does a fuse/circuit breaker blow easier when there is high load on the grid?

[olagjo]

Short version: When there is a high load on the power grid in general, does it make it more likely that my fuse will blow?

Full version: I'm renting an apartment in Rome, where we are currently experiencing a heat wave. When running the air conditioning unit on max, the circuit breaker will regularly trip after an hour or so, taking out the power in the apartment. Since I have to go down to reset it, I know that my apartment is the only one affected, i.e. my power is lost, but no other apartments in the building are affected.

When asking my landlord if he can rectify the situation (e.g. by installing a circuit breaker that can handle a slightly higher current), he says that the power grid is overloaded by people all over the city using their AC in the warm weather, and there is nothing he can do.

I intuitively find it hard to understand that an overloaded grid will lead to my circuit breaker tripping easier, but I don't have a super strong grasp on alternating currents and building/city level grids. Can anyone help me understand, or alternatively make my case?

DISCLAIMER: My main purpose is to figure out if my landlord has tricked me by advertising AirCon when the AC unit in the apartment can't actually be used.

 

[Octopus]

You can't just change a breaker for a higher value one......... It could be that your breaker is failing, so replacing like for like is your only sensible option....

Also what else is connected to the circuit which is tripping?

 

[olagjo]

Cheers! Will it never be the case that the circuit breaker can be "underdimensioned" for what the actual circuit can handle?
That said, the landlord highly likely will not actually make any changes, we just want to know whether there is merit to his reasoning.

This is the main circuit breaker downstairs (labeled with my apt #), so my understanding is that the whole apartment (incl. a fridge, an electrical water heater and of course a bunch of bulbs) is on the circuit.
Is replacing a single circuit breaker a job for an electrician, or can you DIY like you could with a fuse?

 

[olagjo]

(also, my main motivation right now is understanding how this actually works, not necessarily solving the problem I'm a physics nerd with limited real-world understanding of power grids)

 

[Wilko]

Hi - high distribution network load will never cause your mcb to trip quicker. Your mcb should trip at a current less than would damage the installed cable, and so prevent a fire for example. That's why you cannot just change the existing mcb for a higher rated one. At this stage all you can do is run the ac bit less, I think.

 

[freddo]

Isn't the electricity in Italy limited by the suppliers MCB, the rating of which is determined by what contract you have with the electricity supplier? Often a 3KW supply is all that is provided for a normal small property. If you want to use more than 3KW you need to have the supplier arrange a larger supply, which has a larger standing charge. IIRC there are modern meters that automatically cut the supply if too much current is drawn, they are reset by disconnecting all loads at which point the meter restores the power.

 

[static zap]

Some air cons will stall if restarted too soon for pressure to have bled ,
If your problem is brown-outs stalling your compressor .
( have a small incandescent light on and check for brownouts - just before you trip ?)
If trip is thermal you need to find optimum duty cycle + wait time long enough
for aircon to bleed down pressure ! (or not -just theories-I'm not there to observe)

 

[Des 56]

A motor driving a fixed load will react to a lower supply voltage by drawing a larger current,I suppose its possible a lowering of supply voltage during extreme demand could cause a increase in the current taken by the AC motor
If a breaker is passing current near to its maximum full rating,then the breaker is running over its maximum for long enough when supply voltage is lowered it may be long enough to switch itself off through overcurrent

 

[Marvo]

Normally the amount of load upstream elsewhere on the electrical supply grid won't have any effect on whether or not your breaker trips. That said if the high load upstream is causing the supply grid voltage to drop substantially then, as mentioned above, it could cause you air conditioner to draw more current meaning your circuit breaker could trip when normally it wouldn't.

The 'size' or Amp rating of your circuit breaker could be rated according to the size of the wires supplying your flat so you can't just install a new circuit breaker with a higher rating because it would allow the wiring to become overloaded which is a fire hazard.

 

[Pete999]

Some air cons will stall if restarted too soon for pressure to have bled ,
If your problem is brown-outs stalling your compressor .
( have a small incandescent light on and check for brownouts - just before you trip ?)
If trip is thermal you need to find optimum duty cycle + wait time long enough
for aircon to bleed down pressure ! (or not -just theories-I'm not there to observe)

What do you mean by "aircon to bleed down pressure"

 

[Lucien Nunes]

Unlike air compressors that have an unloading valve, aircon compressors are not always capable of immediate restart. If the head pressure (differential across compressor) is still high having just been stopped and restarted, the motor may not have enough starting torque to overcome it as this is usually lower than the running torque. So it will often stall and either trip the MCB or its own thermal protection. Once it has stood for a few minutes, the refrigerant will have continued to flow round the system relieving the pressure and allowing it to restart.

OP- can you take a voltage measurement of the supply when the breaker trips? Do you have other voltage-related problems such as dimming lights? It would be helpful to know exactly what breaker is tripping and what loads it is serving. There could be all sorts of factors at play here.

 

[static zap]

What do you mean by "aircon to bleed down pressure"

Cheers LN .
... Giving air-con enough time for High pressure to disperse ...

 

[Pete999]

Unlike air compressors that have an unloading valve, aircon compressors are not always capable of immediate restart. If the head pressure (differential across compressor) is still high having just been stopped and restarted, the motor may not have enough starting torque to overcome it as this is usually lower than the running torque. So it will often stall and either trip the MCB or its own thermal protection. Once it has stood for a few minutes, the refrigerant will have continued to flow round the system relieving the pressure and allowing it to restart.

OP- can you take a voltage measurement of the supply when the breaker trips? Do you have other voltage-related problems such as dimming lights? It would be helpful to know exactly what breaker is tripping and what loads it is serving. There could be all sorts of factors at play here...

Thanks Lucien,Why do refrigeration compressors stall when switched off and on quickly; or, why do I need to wait three minutes before restarting my air conditioner? - https://electronics.stackexchange.com/questions/115886/why-do-refrigeration-compressors-stall-when-switched-off-and-on-quickly-or-why

 

[The Claw]

Very interesting discussion.

Normally the amount of load upstream elsewhere on the electrical supply grid won't have any effect on whether or not your breaker trips.

Great thinking that low supply voltage will cause an airconditioner to draw more current which could cause an MCB to trip when it otherwise would not. Supply voltage is easily measured.

An MCB or overload circuit breaker should be rated to match the downstream cable size. It's job is to protect that cable from overtemperature which would shorten its working life or even cause a fire. You must not replace it with a higher rated breaker.

Yes, brown-outs stalling the compressor could also cause breaker tripping.

Can you post a photo of the breaker that trips? Is it a MCB or an RCD (safety switch). Airconditioners produce water which can get inside electrics and cause earth leakage tripping.

However my guess at this stage is that it is a simple overload issue. My guess is that running your AC full bore during a heatwave will load the circuit so heavily that when eventually the fridge and/or water heater kick in at the same time, combined with your lights, the circuit is simply overloaded.
Are you able to turn off your water heating when the AC is running? This might solve your problem.