fuses or circuit breakers

[Big COL]

When fitting out my boat my original intention was to use small circuit breakers of the type that can and are incorperated into the 12v distribution panel. I was dissuaded from using this small type of circuit breaker after a friend of mine related the results from tests that had been carried out on this type of circuit breaker.

The test showed that the tripping rate varied from 20% to 60% on brand new breakers, and increased with age. Worst case senario a new 20 amp breaker could take a load of 32 amps before tripping. Some of the test showed that a small percentage tripped under the breakers rateing. Reluctantly I reverted to using automotive blade fuses although I am aware of the fact that fuses degrade with age, but at least it's on the safe side. I have had no problem whatsoever with the fuses and the finished article is neat and tidy. However I have always felt dissapointed that I couldn't build the panel I had originaly designed. Has anyone had problems with these small breakers, this may be a difficult question to answer because if you say I have used these for years and they have never tripped then it may be due to the fact that they are carrying more current than designed for. Catch 22.

[John Orentas]

Colin.

 

Personally I have always had a preference for fuses, on the grounds of simplicity, I always have the feeling "what if the breaker itself was defective?", how would you know if there was a genuine fault or not. Having said that circuit breakers do seem to be very reliable so perhaps it is just 'me'.

 

Don't get to hung up about the accuracy of breakers or fuses, they are probably equally inaccurate and it doesn't really matter, they are there only to protect your wiring, if a 20 amp fusing device trips or blows at 15 or 30 amps, in practise in 99 cases out of 100 it doesn't matter.

 

You should rate your device for the wiring in the circuit and not relative to the equipment or appliances they are supplying.

 

If you decide on fuses find a nice range of industrial fuses and carriers, much less fiddly.

[DHutch]

We use orginary 5A 240v breakers in a normal 240v disbox (and MK sentry to be exact)

 

Daniel

[NB Willawaw]

Sorry, Ive only used fuses for DC and trip breakers for AC mains distribution.

On the DC side, I tend to use high value industrial fuses for heavy duty items and one or two individually fused DC distribution panels for max 10A circuits.

[ChrisPy]

We use orginary 5A 240v breakers in a normal 240v disbox (and MK sentry to be exact)

 

Daniel

<{POST_SNAPBACK}>

for 12VDC ?

 

I believe there are health warningsa about using the contacts found on AC devices failing in DC service. Are these warnings just alarmist?

[stuart]

for 12VDC ? 

 

I believe there are health warningsa about using the contacts found on AC devices failing in DC service.  Are these warnings just alarmist?

<{POST_SNAPBACK}>

 

I've also read this and I think that the issue is true, however its one of those things that would take ages to fail in the real world.

 

Not sure what you meant about "health warnings" !!

[DHutch]

I've also read this and I think that the issue is true, however its one of those things that would take ages to fail in the real world.

 

Not sure what you meant about "health warnings" !! 

 

It was apparently suggested by a friend of my granfarther, he was also dubious, but tested it by loading the end of each proposed cable run and see if it triped at a suitable time (ie well before ythe cable warmed up) and it did. and if thats not good enought, what is?

 

- My grandad was happy with it, and that will do for me.

 

 

Daniel

[John Orentas]

Most circuit breakers will have for example a rating of "up to 28 Vdc or 250 Vac". It is better to buy breakers with 'dc' mentioned in the specification.

 

The restriction is generally against high voltage direct current systems which are rare even in industry, for voltages less than 30 Vdc there is rarely a problem. It is all to do with switching speeds, contact gaps and arcing of the the current during the switching.

 

When buying switches it is as well to look at it's specification some have may have an amp. rating very different from 230 volts and 12 volts.

 

In practise however we tend to grossly over rate electrical equipment on boats things such as light switches may well be rated at 10amps when it is to be used to switch a 1 amp lamp. If you purchase a relay for your split charge system it may well be rated at 60 amps with a life of 5 million operations, it will last a long time.

Edited April 29, 2005 by John Orentas

[Guest]

We have used ordinary domestic 240 v type breakers for 12v dc. BSC man said he prefers these type. I once picked up a DC panel in a chandlers only to read on the switches"ac only".

[John Orentas]

We have used ordinary domestic 240 v type breakers for 12v dc.  BSC man said he prefers these type.  I once picked up a DC panel in a chandlers only to read on the switches"ac only".

<{POST_SNAPBACK}>

 

I bet if you asked whoever printed "ac only" on them he could not give you a rational explanation why. Other that "well they have only been tested on ac".

[Big COL]

I bet if you asked whoever printed "ac only" on them he could not give you a rational explanation why.  Other that "well they have only been tested on ac".

<{POST_SNAPBACK}>

 

John

 

There must be some truth in this ac only statement, but like you I can not see why. To limit sales to ac only applications, manufacturers are missing out on sales.

which makes me think there must be a reason for this.

[John Orentas]

Colin.

 

There is of course a technical difference between ac & dc switching, all to do with inductive and resistive loads most of which goes over my head. What I can never understand is that on some equipment the current ratings differ by a factor of 10 but with others they are the same. However as I have said before, with boats we are in the fortunate position that we can massively 'over spec'. all our equipment so things are never critical.

 

With the circuit breakers, I would not be surprised if it is simply that the makers have never bothered to submit their product for the appropriate approvals, imagining that there was no requirement for a dc product.

 

There are of course lot's of breakers that do have a dc specification, might be as well to use them, you may get a particularly picky surveyor.

Edited April 30, 2005 by John Orentas

[jollyroger]

It's all to do with arcing, dc arcs very eisily, ac doesn't

[DHutch]

It's all to do with arcing, dc arcs very eisily, ac doesn't

 

He speaks the truth.

 

I knew a house that had a DC electrical system, you should have seen the sparks on the (admitedly now old) light swiches.

- They really where quick big!

 

 

Daniel

[Alan Saunders]

. . . I knew a house that had a DC electrical system, you should have seen the sparks on the (admitedly now old) light swiches.

- They really where quick big!

Daniel

<{POST_SNAPBACK}>

Thnks for that Daniel. As I remember it the USA at one time had 110v DC supplies to houses - mainly for lighting. Various problems ensured that it was replaced by AC but the company supplying DC (Edison?) argued that it was superior.

 

Back to the AC/DC switch rating question. Switch terminals fail because of:

1) Corrosion - fail open circuit/high resistance

2) Arcing - can fail short circuit (welded together).

 

To reduce corrosion and contact resistance you can use a noble alloy but it will have a low melting point. To reduce damage from arcing you can use an alloy with a high melting point but it will liable to corrosion. Contacts can be designed with a wiping action to remove corrosion but it is difficult to achieve this simply and snap the contacts apart fast enough to minimise arcing.

 

Breaking an inductive load induces voltages far greater (10 or 100 times) than the supply - thus the arc suppression capacitor on the 12v spark ignition engine contact breaker points.

 

Once an arc is established the air is ionised and a lower voltage is required to maintain (or initiate) the arc. Reversing the current will break the arc if the gap is sufficient. If you can get the contacts a good distance apart within 100th of a second (at 50hz) the arc will cease.

 

Switches can be expected to have a life of tens of thousands of operations whereas contact breakers may only be operated a few hundred times in their life. The short-circuit failure mode is unacceptable for a contact breaker; better that it self-destructs!

 

So far as I know, it is not a case of obtaining approval; some AC switches are totally unsuitable for DC operation. There may be AC switches that are suitable for DC operation but the designer was not made aware of that requirement.

 

NB Just because 240v 50hz AC CBs have worked fine on 12v for 20 years does not prove that they are adequate. I will stick with fuses.

 

Alan