Unnecessary Fused Disconnect? 2

[jedijunk]

Ok, simple question here. . . just need to make sure I'm not missing something obvious. I'm not a trained electrician, but I'm the best I got here in West Africa. Ha ha. This question pertains to a system supposedly set up by a professional electrician, but I suspect he wasn't familiar with the kind of sitation we have here.

I've got a 2kw 3-phase generator powering a facility here. There's a 50amp breaker on the generator. From there the three phases run through an emergency disconnect box where each phase is fused with a 100amp blade fuse. From the emergency disconnect box the three phases run into my master panel, which has a 40amp master breaker.

Question: is the fuse portion of the master disconnect box functionally unnecessary (there's no electrical code here)? Inline between a 50amp breaker and a 40amp breaker, I simply cannot conceive of a situation where those fuses will blow. I'd like to replace the box with a simpler emergency disconnect switch. This is a closed system which at no point ties into public utility power.

Thanks in advance!

 

[dpc]

Putting aside any concerns about local codes, I think the main thing you need to worry about is safety.

How far is the generator breaker from the disconnect switch? Normally if a feeder enters a building from outside or another building, it is a good idea to provide a LOCAL disconnecting means. This encourages its use (saves a walk) and is more visible to whoever is doing the work.

If the generator breaker is nearby, accessible, and lockable in the OPEN position, you might be able to live without the fused disconnect. Generally the cost of a fused switch is not much greater than a non-fused disconnect. The fused switch will have much higher short circuit rating than the non-fused switch.

The electrician was probably just playing it safe - who knows what will be connected to this in the future - a much larger replacement generator, a generator without a main breaker, etc, etc. I would have probably done the same.

 

[jedijunk]

Thanks, dpc!

That makes sense. Distance between generator and my master disconnect/master panel is literally three feet (same room). So distance isn't an issue.

Lots of things will need to be changed if we ever go to a much bigger generator (or if the country of Liberia ever joins the 20th century and gets grid power!). The main thing I wanted to verify is that the fuses aren't likely to be needed in the current configuration. Star for that. . .

 

[itsmoked]

To add to dpc's nice response I would suggest the electrician had a fused disconnect and so used it. I often see fused disconnects with pieces of copper pipe in them converting them into 'just' a disconnect. Fused ones seem to often be more common than non fused ones.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

For a 2kw generator, everything seems to be over protected by a factor of maybe 8 or 10.
At 125% I would use a 50 amp breaker to protect a 14.4 kw generator at 120/208 volts.
Power factor is usually 100% at this size.
I suspect that your best protection will be stalling the motor in the event of a short circuit. Sometimes "best" is not very good.
The 50 amp breaker on the generator will give you a disconnecting means at the generator but not much protection.
The 100 amp switch seems to be redundant, if it is within 3 feet of the generator. One issue that probably isn't important is the ability to lock the switch in the off position. This is important in the US but is probably not an issue in the third world.
Short circuit current is not really a serious issue with a 2kw generator. I would be surprised if the short circuit current got much over 60 amps if that much.
Just to make sure there are no misunderstandings, are you sure you have three phase? It is not common on gen-sets that small. Three wire 120/240 volt systems have three wires but are single phase systems. The center wire, or neutral wire should not be fused, and is best not switched.

A three phase panel will have three bus bars to which the breakers connect. There will probably be a separate terminal strip to connect the neutral conductors. The main breaker will be three pole.
As you go down the panel, every third breaker will be common on the same bus bar.

A single phase panel will have two bus bars to which the breakers connect. There will be a separate terminal strip to connect the neutral conductors. The main breaker will be two pole.

As you go down the panel, every second breaker will be common on the same bus bar.

With the size of your breakers and the small size of your gen-set, it doesn't much matter if it's three phase or single phase. The fuses and breakers are still grossly oversized.
What are you voltages and are the breakers 2 pole or 3 pole?
respectfully

 

[jedijunk]

itsmoked (love the username!),
Good point. That's highly likely.

waross,

2kw was a typo in my original post. I saw it after I already got my first response, but I knew somebody was gonna catch that. It's a 20kw genset. So yeah you're exactly right - it would be overprotected by a factor of 10 for a 2kw genset.

And yes, it's a 3-phase genset, but that's of course no surprise when you know it puts out 20kw. It actually runs into a European-style master panel which uses "pass-through" breakers (current in one side and out the other) instead of rails. Actually makes it very easy to see visually which breakers are on which phases. We have the genset tuned to run 120v per phase (dropping to 112-115v under normal working load). Thanks for a helpful and informative response - star for that!

I'm a computer programmer by trade that landed in the middle of nowheresville Africa and became the go-to electrician by way of the fact that I have a mind for technical things and know how to find answers on the internet (an internet-trained electrician - now THERE'S a scary thought!). I have a healthy respect for 220V current and for second opinions, so you'll probably see me in here from time to time asking questions like this.

In a country devoid of standards, code, or even true expertise, it's amazing what responsibilities are placed on the shoulders of guys who have a decent measure of intelligence and common sense. But believe me - I spent three weeks studying 3-phase power, volts/amps/watts, neutral vs. ground, 50hz vs 60hz, and the exact setup of our particular facility before I ever went NEAR a breaker panel. If I die in Africa, I want it to be from malaria (fun experience - recommend it highly!) or attack by some exotic animal - not because I was too stupid to learn what I was doing before I started working with 220v power!

 

[itsmoked]

What I find amazing is no power infrastructure but here you are communicating with us over the internet.! How?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jedijunk]

Keith,

It's a bit complicated, but you asked! At the moment the genset isn't running. So my laptop is being powered by an inverter, which is in turn being powered by a bank of four deep-cycle batteries (that charge when the genset is on). (Incidentally, the batteries are also powering a little 12vdc flourescent light so I can see and a great little Papst muffin fan so I don't die of heat exhaustion) The inverter is also powering a wireless router and a slick little box called the Navini Ripwave that establishes a wireless connection (I think at 2.4ghz) to a cellphone tower half a mile away (which is, of course, powered by a 24-hour genset). That relays my signal to the ISP's headquarters, where yet another genset powers their servers and satellite link to the outside world. Thanks to the cost and relatively limited throughput of a satellite link, my connection speed is ostensibly 64k, although on an average day it's actually somewhere south of 30k, for which priviledge I pay US$120/month.

Moral of the story: enjoy and be grateful for you 24-hour power and your cable/DSL connection, boys and girls! Like most Americans, I never gave them much thought until I moved here!