MCB Trip thingy

[mrsmelly]

Having just read a thread from another poster re his electrical problems and having myself had an MCB blow recently and replace it after advice on here I feel I need to ask a question which is safety related for my peace of mind. So I happen to know from experience that gas doesn't always fail safe and indeed when the regulator fails it can indeed end up with the boat filling up with gas. When my MCB failed it tripped and would not reset so in fact it failed safe as no leccy getting through. My question therefor is do they always " Fail safe "? or can they indeed melt or something inside and become permenantly on?

Thanks

[WotEver]

MCBs can indeed fail with the contacts welded together but it is not something which generally occurs while it’s just sitting there passing current. It usually occurs when opening the breaker while a heavy load is running, causing an arc across the contact pads. It’s FAR more usual for them to fail ‘safe’. 

Take a look at this:

https://support.industry.siemens.com/cs/document/22979862/reasons-for-contact-welding-in-contactors?dti=0&lc=en-WW

[Tony Brooks]

And its why it is unwise to use only AC rated MCBs on DC supplies, even though the voltage may be 20 times higher. DC will arc more than AC.

[mrsmelly]

Thank you both. Promptly answered

[ditchcrawler]

1 hour ago, WotEver said:

MCBs can indeed fail with the contacts welded together but it is not something which generally occurs while it’s just sitting there passing current. It usually occurs when opening the breaker while a heavy load is running, causing an arc across the contact pads. It’s FAR more usual for them to fail ‘safe’. 

Take a look at this:

https://support.industry.siemens.com/cs/document/22979862/reasons-for-contact-welding-in-contactors?dti=0&lc=en-WW

Now we had a major problem offshore in my early days with breakers, They were specified double pole  so that the circuit was completely isolated but we had quite a spell of one terminal welding closed, more apparent when its the live one, breaker off and lights off but supply still live

[WotEver]

36 minutes ago, ditchcrawler said:

Now we had a major problem offshore in my early days with breakers, They were specified double pole  so that the circuit was completely isolated but we had quite a spell of one terminal welding closed, more apparent when its the live one, breaker off and lights off but supply still live

Did you ever determine the cause?

[ditchcrawler]

1 minute ago, WotEver said:

Did you ever determine the cause?

No, we changed them out. too long ago to remember the make but German I think.

[WotEver]

1 minute ago, ditchcrawler said:

No, we changed them out. too long ago to remember the make but German I think.

So effectively the ‘cause’ was cr@p breakers?

[frangar]

Ive also had them so they trip at far less than the rated load....this seems to happen when they are regularly tripped due to circuit overloads...happens quite often on temporary power installations where caterers seem to think they can power an all electric kitchen but only need to order a 10 amp supply. 

[WotEver]

1 minute ago, frangar said:

Ive also had them so they trip at far less than the rated load....this seems to happen when they are regularly tripped due to circuit overloads...happens quite often on temporary power installations where caterers seem to think they can power an all electric kitchen but only need to order a 10 amp supply. 

At a place I worked in the 70’s I discovered one of our missing stage weights. It was tied to the 100A kitchen breaker with a bit of string to stop it popping!

[ditchcrawler]

3 minutes ago, WotEver said:

So effectively the ‘cause’ was cr@p breakers?

Quite possibly, I can't think of any other reason

 

[frangar]

Just now, WotEver said:

At a place I worked in the 70’s I discovered one of our missing stage weights. It was tied to the 100A kitchen breaker with a bit of string to stop it popping!

LOL.....it’s always the caterers!!!

[mrsmelly]

10 minutes ago, frangar said:

LOL.....it’s always the caterers!!!

A dear friend of mine has a pub that in the summer is very busy and she has all her stuff switched on at once. For many years until she had the place rewired only last summer the distribution board up behind the bar had the door open and she ran an electric fan at it to keep it cool to stop everything blowing!! I kid you not. I did tell her that the fan was adding to the problem but she informed me that it did help!! 

[Taslim]

Yea. It would have tripped 'else.

[TheBiscuits]

6 hours ago, mrsmelly said:

A dear friend of mine has a pub that in the summer is very busy and she has all her stuff switched on at once. For many years until she had the place rewired only last summer the distribution board up behind the bar had the door open and she ran an electric fan at it to keep it cool to stop everything blowing!! I kid you not. I did tell her that the fan was adding to the problem but she informed me that it did help!! 

I have seen that too, at a place run by a very good electrical engineer (yes, a real one not a technician!).

He pointed out that

1) the few tens of watts of the fan motor were well worth the tradeoff for the forced air cooling effect on the fuseboard

and

2) would I suggest spraying water on the fuseboard as an alternative?

Both of which are very good points!

He did get an upgraded supply as soon as the local electricity board would do it though.