Fried Circuit Breakers.... Help

[stud858]

1 hour ago, RocketDog said:

As supply voltage drops the rotor slows and effectively lowers the inductance of the field winding. This lowers the inductive impedance causing higher current draw

I thought the inductance of a coil is calculated from its number of turns, diameter of turns and the length only. Perhaps now I understand that's only true when no other em field is interacting and it's the induced field in rotor which is effecting things by lowering the inductance of field winding.

It just seemed like a counter intuitive thing.

Those pesky invisible em waves giving me headaches.

 

 

 

 

 

 

[sometimewoodworker]

7 hours ago, RocketDog said:

At 10A you must be assuming the internal triac temperature control circuit is in regulation mode. 

If it's on full tilt boogey it will draw closer to 30A for a 6kw heater at 220V. This should be OK indefinitely for a 32A breaker. No? 

I am not making assumptions, I'm giving observations of fact, as you can see from my other post.

 

No, on full power 48C it is drawing about 20A, at the moment, so a 20A breaker will almost certainly be sufficient (even if you get it drawing 27A) as they do not trip quickly on short term over current situations

[RocketDog]

4 hours ago, sometimewoodworker said:

I am not making assumptions, I'm giving observations of fact, as you can see from my other post.

 

No, on full power 48C it is drawing about 20A, at the moment, so a 20A breaker will almost certainly be sufficient (even if you get it drawing 27A) as they do not trip quickly on short term over current situations

Ok. I'm not arguing. Just saying 6kw/220v=~27A as you say.

 

So I'm guessing the heater manufacturer is playing games with their specs and the heater is actually not 6000W.

 

If the triac is full-on and the load is purely resistive, then your ammeter is measuring true RMS sinewave current. Once the triac starts chopping each half cycle the waveform won't be a sinewave any more and the ammeter calibration won't be correct. 

I guess it's also possible that the line voltage is lower than 220 but it would have to be about 25% low to fit the numbers. One of the voltage, current, or power numbers is inconsistent. 

 

I think we're seeing some marketing games here. 

 

At any rate, you're right, the breaker will handle it. 

[RocketDog]

6 hours ago, stud858 said:

I thought the inductance of a coil is calculated from its number of turns, diameter of turns and the length only. Perhaps now I understand that's only true when no other em field is interacting and it's the induced field in rotor which is effecting things by lowering the inductance of field winding.

It just seemed like a counter intuitive thing.

Those pesky invisible em waves giving me headaches.

 

 

 

 

 

 

You are totally correct as far as you go. There are other factors such as closeness of windings, wire resistance, eddy current losses, and how many layers are wound and the insulation between windings since each successive layer is farther from the core, etc.

 

And that's the other big factor: core material and lamination, etc. Theoretically transformers seem like simple devices, but in practical terms transformer designers are very well paid individuals. 

 

 

The issue with the motor is that as the slip increases due lower field strength due to lower voltage then the rotor is not as closely coupled to the field, and it's this coupling that produces the torque. As the inductive reactance goes down from poorer coupling to the field the winding becomes more purely resistive and thus draws more current. AC current is limited by the sum of winding resistance plus the inductive impedance. A DC current would only see the winding resistance. 

 

Think of the motor as a transformer with a rotating core carrying a shorted secondary winding and you get the idea. 

 

Because the impedance is also a function of frequency, transformers or motors designed for 60Hz will run hot in Thailand at the 50Hz line frequency due to the lower impedance drawing more current. 

 

I learned this lesson as a kid trying to use WWII military aircraft electronics with transformers designed for the 400Hz AC systems they used. Connecting these to 60Hz current promptly burned the transformer out. They used 400 Hz because magnetics become more efficient at higher frequencies less core material, think weight, is necessary. 

 

For the same reason modern switching regulators like computer power supplies operate in the MHz range. 

[Fruit Trader]

2 hours ago, RocketDog said:

If the triac is full-on and the load is purely resistive, then your ammeter is measuring true RMS sinewave current. Once the triac starts chopping each half cycle the waveform won't be a sinewave any more and the ammeter calibration won't be correct. 

If his meter is capable of true RMS then readings will be accurate even when the control circuit begins phase angle switching and modifying the waveform. There is a good chance the readings are being taken with a cheap or older averaging meter.

 

As you have already indicated, equipment labelled 6kw full load should be protected with a 32A breaker. Using the over current area of a 20A circuit breaker is just bad practice and likely to cause unnecessary heating of a breakers bimetallic circuit.

 

 

 

 

 

 

 

 

[RocketDog]

8 hours ago, Fruit Trader said:

If his meter is capable of true RMS then readings will be accurate even when the control circuit begins phase angle switching and modifying the waveform. There is a good chance the readings are being taken with a cheap or older averaging meter.

 

As you have already indicated, equipment labelled 6kw full load should be protected with a 32A breaker. Using the over current area of a 20A circuit breaker is just bad practice and likely to cause unnecessary heating of a breakers bimetallic circuit.

 

 

 

 

 

 

 

 

Agreed. It's even possible that the meter uses the older D'Arsoval analog meter movement, which is a DC device. So there is a shunt and rectifier involved as well, both adding to inaccuracy. 

I'm guessing that today any D'Arsoval movements sold are extremely cheaply made.

 

In the old days precision movements used in military electronics could cost over 100$, and those were old dollars at that, when currency was backed by real metal. Sigh....... 

 

But, it's also possible that he has a true RMS digital Ammeter. At any rate I still think the heater manufacturer is fudging the power rating. . Maybe he's using a 'peak' power spec, so common with stereo amplifier vendors to wow the audience with smoke and mirrors.

 

As an electronic design engineer I have learned to read even integrated circuit chip spec sheets with a jaundiced eye. 

 

Good point about fatiguing the bimetal element. I've replaced the often worthless thermal 'fuses' in kitchen appliances too often. In this case I believe it to be planned obsolescence, pure and simple. Very few folks are willing to probe the guts of their appliances, understandably. Ah, modern life! Love it or leave it. 

 

I recently just shorted one in a rice cooker I bought at Big C that failed 2 months after I bought it. In some cases like toasters and small ovens the heater circuit wiring is done by spot welding steel rails, making solder repair impossible. I know I could silver solder it with a small torch, but just give up and toss the thing instead. 

Onward thru the fog. 

[sometimewoodworker]

On 3/9/2019 at 8:01 AM, RocketDog said:

But, it's also possible that he has a true RMS digital Ammeter. At any rate I still think the heater manufacturer is fudging the power rating. . Maybe he's using a 'peak' power spec, so common with stereo amplifier vendors to wow the audience with smoke and mirrors.

Just FWIW

Background load of about 20 lights a couple of refrigerators computers and routers 

 

adding the water pump for a single shower 

 

Adding the shower on 33C

 

changing the temperature to 48C which also pulls the voltage down by 10V

 

changing the temperature to a more usual 36C

 

[RocketDog]

Yep. So it's obviously not a 'bang-bang' controller. It's modulating power with scr or more likely triac device. Your initial 229VAC with light loads is close to the nominal 230, So you have a good line to the house. 

The voltage drop under heavy load is wiring loss as heat. What wiring it is depends on where you have the voltage and current/power meter attached. Likely it is the wiring from the breaker box load center to the shower. 

So as your meter indicates, with the shower on highest temperature setting, very little phase angle chopping, you're seeing about 4,000W.

I suppose it could be 6kw heater element but if so, the controller won't allow that much power to it. Of course the power necessary to maintain a given shower temp depends on the temperature of the incoming water. Hotter incoming water means lower power to raise the shower temp. 

[sometimewoodworker]

19 hours ago, RocketDog said:

The voltage drop under heavy load is wiring loss as heat. What wiring it is depends on where you have the voltage and current/power meter attached. Likely it is the wiring from the breaker box load center to the shower. 

It (the meter) is measuring the main feed from the meter to the house. So it is possible that the main supply cable has been undersized. I will need to check on the voltage at the meter then quite possibly double up on the main feed from the meter if the voltage isn't dropping at the meter.

 

Though I doubt that it will be cable loss (the supply is only 40 metres and is copper) and think that the load is probably pulling the whole phase down so we may have to talk to the PEA 

 

AFIK both of the shower circuits are dedicated 4mm direct runs.

 

FWIW with both showers running the mains voltage is being pulled down by 25v. This makes an overloaded or too long Phase more likely. I doubt that the wiring in the house is bad enough (I think it is actually good)

[RocketDog]

8 minutes ago, sometimewoodworker said:

It (the meter) is measuring the main feed from the meter to the house. So it is possible that the main supply cable has been undersized. I will need to check on the voltage at the meter then quite possibly double up on the main feed from the meter if the voltage isn't dropping at the meter.

 

Though I doubt that it will be cable loss (the supply is only 40 metres and is copper) and think that the load is probably pulling the whole phase down so we may have to talk to the PEA 

 

AFIK both of the shower circuits are dedicated 4mm direct runs.

Agreed. If 4kw pulls the phase down that much then I would speak to PEA. However, I guess it's possible your neighbors are using so little (I'm the power pig on my Soi) that your relatively light load just brings it down from floating. Still, I can't really buy that argument myself. 

They might tell you if it's above 220 then they won't pursue it. Let me know what happens. 

Good luck. 

[sometimewoodworker]

4 minutes ago, RocketDog said:

Agreed. If 4kw pulls the phase down that much then I would speak to PEA. However, I guess it's possible your neighbors are using so little (I'm the power pig on my Soi) that your relatively light load just brings it down from floating. Still, I can't really buy that argument myself. 

They might tell you if it's above 220 then they won't pursue it. Let me know what happens. 

Good luck. 

I was testing while you were writing and both showers on pull the power from 225v down to 200v

[RocketDog]

So if both showers are on full heat, then from previous post each draws about 20A. You are seeing a ~30V drop. So V/I=R. 30V/40A=0.75 ohms total resistance in the two conductors, round trip so to speak. So each conductor is ~0.375 ohm for a 40m run. You can decide if this is a valid resistance and/or actually determine the cross section area of the wire using these tables. 

http://www.interfacebus.com/AWG-table-of-different-wire-gauge-resistance.html

Off the top of my head I'd guess that this is probably too high a resistance for the primary wiring going to your house. 

You can use the formula above, Ohm's law, and careful measurements to make a determination. 

I'm pretty sure your meter is reading RMS  values so you can treat it as a simple DC circuit and use Ohm's law as above.

At least if you calculate the primary wiring to be too small you can make your arguments with math and not just handwaving. 

 

Two hundred volts seems way too low for a 40A load. It's possible that their transformer is too small for your area loads, which would also account for the line sag.  In fact, it seems likely as I think about it. If you take measurements at 2 different heater currents and then calculate two grossly different wiring resistances then you know it's really a line sag problem because the wiring resistance doesn't change with voltage /current. It's a function of the wire cross section and the resistivity of copper, both of which don't change. 

 

To be positive about where wiring loss occurs just turn on one shower and take voltage/current readings at the breaker box and then at the power pole if you can get at it.  You only need one current reading because it will stay constant and you will only need to see the voltage change at each end of the cable. On my Soi it is dangerously possible to get a reading directly at my meter! 

You can do the same test at the breaker box and then the shower heater to calculate the house wiring resistance. 

 

Feel free to PM me if you want. 

Good luck. 

[bwpage3]

Convert the watts to amps, and check if the electrical current load in amps is less than the breaker size in Amps

 

6000 watts at 220 VAC should be around 27.3 amps

 

That is how much current in amps the circuit will be using

 

Wire size and circuit breaker should be sized accordingly

 

A 30 amp breaker should be used with 10 gage copper wire or 8 gage aluminum wire

[sometimewoodworker]

13 minutes ago, bwpage3 said:

A 30 amp breaker should be used with 10 gage copper wire or 8 gage aluminum wire

I've no idea about wire gage sizes as they are not used here AFIK it's only in the US that the world wide standard sq mm sizing is not used.

 

However the MCB and wiring is correctly sized, also 30A MCBs are not common it is 32A