Just my luck

[Steve]

Our shop is closing down in august. Yesterday I realised the stock in the freezers was getting very low, so being energy conscious, I emptied one freezer, so the other one was totally full. I then switched off the sockets supplying the empty freezer. All good so far.

I will pause to explain the nature of the freezers. They are upright glass doors. Each cabinet has 3 doors, there are 2 cabinets. For each door there is a compressor. All 3 compressors engage at once via a LCD control and relay, and there is a lead and plug for each compressor.

I came in this morning, and shock horror, there are no signs of life in either freezer. Thankfully the full freezer is still cold. I turned its MCB back on. The lights came on, and the LCD. Then the click of the relay as the compressors engage, and the MCB tripped. I also tried turning off the other 2 plugs, so just the controller and the 1st compressor would come on, but the fault is on that plug, as it tripped again. I could see the relay engaging and going back as it tripped. So I fired up the other freezer again and transfered the stock over

Engineer coming sometime today they said. So just how much current does it take to trip a C32 instantly? What is likely to cause this fault? Compressor or relay? Answers on a postcard!

 

[ricicle]

C32 trips instantaneously with 307A @ 230V

 

[Tim 77]

C32 trips instantaneously with 307A @ 230V

Is there a calculation for that?

 

[Steve]

Must have been a dead short then. Is this possible with relays? Its a 10 amp 250V 2 pole relay which plugs into a base.

 

[ricicle]

C32 trips instantaneously with 307A @ 230V

Is there a calculation for that?

Yeah - Ohm's law

No seriously, from Table 41B2 (h) in BS7671 the maximum EFLI for a type C 32A is 0.75 ohms for instantaneous trip @ 230V

230/0.75 = 306.6666666667A

 

[Tim 77]

Ah, cheers Ricicle

 

[Adam_151]

It makes more sense with 240v..

Type C... between 5 x In and 10 x In, for for 32A device... between 160A and 320A so to be sure of disconnection we have to aim for the top end, which is 320A

Which is what we get if we do 240/0.75

 

[fattony]

I came in this morning

Friday the 13th

 

[JohnD]

Our shop is closing down in august.

Does that mean you will be cast aside like a used prophylactic?

Into the skip of redundant humanity?

 

[Spark123]

C32 trips instantaneously with 307A @ 230V

Is there a calculation for that?

Yeah - Ohm's law

No seriously, from Table 41B2 (h) in BS7671 the maximum EFLI for a type C 32A is 0.75 ohms for instantaneous trip @ 230V

230/0.75 = 306.6666666667A

I didn't think the tripping current of an MCB changed with the voltage
Think the magnetic trip setting for a type C (5-10x) is just somewhere between 5x32A=160A and 10x32A=320A.
The max efli thing (although irrelevant to craftys problem) is calculated using Uoc which is 240v, is simply the Uoc divided by the maximum current for instantaneous trip i.e. 240/(10x32) = 0.75ohms at cable max operating temp. I know the IEE mix up the two figures, though for a Uoc of 240v the Uo is 230v!
Now I've just read adams post bah!!

 

[ricicle]

I didn't think the tripping current of an MCB changed with the voltage
Think the magnetic trip setting for a type C (5-10x) is just somewhere between 5x32A=160A and 10x32A=320A.
The max efli thing (although irrelevant to craftys problem) is calculated using Uoc which is 240v, is simply the Uoc divided by the maximum current for instantaneous trip i.e. 240/(10x32) = 0.75ohms at cable max operating temp. I know the IEE mix up the two figures, though for a Uoc of 240v the Uo is 230v!
Now I've just read adams post bah!!

Yeah, I just jumped straight in with two feet, before remembering the 10x trip for Type C is based on 240V, not the 230V as stated by the table

 

[Spark123]

An MCB operates on current, not voltage
The maximum loop impedance tables are just to do with ohms law, if the tabulated value is exceeded then there is a possibility there will not be enough current flowing in a circuit for the MCB to disconnect in 0.1s.
They just put 230v into the table to confuse matters.
The earth full fault loop includes the circuit protective conductors, suppliers earthing arrangement, suppliers transformer and the phase conductors. Although the supply entering the premises is in theory 230v (Uo), the DNO transformer is still in theory 240v (Uoc) which is the voltage causing the current to flow around the fault loop.
I believe the next edition of BS7671 may use 230v in the calculations, just to keep europe happy anyway.

 

[Steve]

FYI, twas a faulty compressor shorting to earth on startup. Now need a new compressor and the associated brazing etc. I hope they send a different guy to last time, he managed to melt the floor and filled the shop with acrid smoke. He said it was normal. I've seen a few compressor changes, never seen this happened.

For some bizarre reason the fridge guy left the freezer running on 2 compressors (after disconnecting the faulty one). It cannot get down to temperature (set at -23, lowest yet is -14, too warm), and the compressors cut out on their safety timer after a few hours of trying. Theres nothing in the freezer, but he left a note saying not to turn it off or it will blow up. Way to waste our energy, looser.

This is another potential problem with these freezers to add to the growing list. Quite a few of our stores have them. They get clogged up with ice (4 days defrosting worth), compressors fail often, refrigerant leaks, they dont like moving, circuitry burns out, doors heaters go short circuit, compressors go short circuit, defrost heaters fail, temperature sensors fail . . . . you get the idea!!!

Strangely, there are four light tubes in each cabinet, I have NEVER seen one fail. The tubes are the most reliable part of these units.

 

[r.bartlett]

Who do you work for and who are the Fridge contractors?

Seems like there's scope for a professional outfit unless they're hired on price first of course

Cheers

Richard