breaker/fuse protection

[stevo42]

I could use some advice on protecting my emergency power system which is now, finally, up and running. I have a Lister SL2 (8.5 HP) powering a ST5 generator head and a Lister SL1 (4.25HP) powering a Northern 2 pole, 2.9 KW head. both wired 240/120. I'd  run the SL1 when I don't need the well pump or shop tools as its so easy on fuel and the SL2 when I need a little more power. I'd  rather have the breaker/fuse trip under a load that gives me a good margin than come close to over loading either gen set or engine of either set. I've been running the SL1 for years without any such protection but since I've added the SL2 and overhauled the system and have been reading this forum I've learned CBs or breakers would a good idea.  BTW, I got a qualified marine electrician friend to go through my whole system from my main 200 amp panel, six circuit transfer switch and back to the gen sets. What I need to add now is either CBs or breakers that gives a healthy margin for protection in case someone turns on the air compressor or table saw in the shop.

[dieselgman]

We use 40 amp 2-pole breakers on our very small gensets... mounted right at the output leads they can protect your set and everything downstream from overloads and short-circuits.
A very thorough grounding system is imperative for personnel (as well as equipment) protection. You want your generator main housings and generator frames bonded securely to earth (ground rod), as well as all other wiring runs, metallic housings and powered equipment. Good grounding will protect from (or reduce) lightning strike damage and it is a necessary part of system protection even ahead of circuit current overload protection.
The vibration as well as rotational and environmental stresses right at the set are a serious source for insulation failure... very solid and even redundant grounding paths are recommended and also a primary point for routine maintenance testing.

dieselgman

[stevo42]

I drove a good grounding rod under the roof drip line of the new engine room and grounded everything to that. Are you saying to use 40 amp breakers for my 2.9 KW gen head and the ST5?

[dieselgman]

40 amps for the 5kW is about right, 25 amps for the 3kW would be closer match to the smaller unit...

As far as actual current overload on your generator head, the best and most practical design option is to oversize the generator head so that you will run out of horsepower before running out of current capacity in your windings. This should be the case with 8 1/2 horsepower and 4 1/4 horsepower inputs respectively. One reality is the fact that the factory numbers rarely reflect actual precise performance when it comes to horsepower. On a dyno you will almost always fall short of the ideal numbers and this gap will increase as the machine gradually wears and loses power/performance.

dieselgman

[stevo42]

OK, thanks, I have a 30 amp "push on, fused safety switch" If I used a 20 amp slow blow fuse on each leg in the switch, wouldn't that work for the ST5 and I could get another switch and use 12 amp fuses for the 3 KW set?  That seems like a simple solution to protecting my gen sets. My usual electrical needs are well below the potential output of my sets so I would be tempted to use 15 amp fuses for the ST5 just to be sure and 10 amps for the 3KW.  If these fused safety switches will do the job installed first thing on the 240 line right out of the gen sets that will be easy to install.

[dieselgman]

Fuses will work fine... perhaps less than ideal because they are one-time-use items. Circuit breakers will most likely outlast your set and can also function as disconnect switches.

dieselgman

[stevo42]

Good, thank you for the help. I should be set for another northwest winter.  Are parts still available for these SLs should I ever need any? I have the fuel filtered by a Racor filter/water separator.

[buickanddeere]

  There is short circuit protection and there is thermal protection.
   Don't mix them up as it's legal to install a 90 amp P fuse, a 50amp time delay D fuse or a 70amp breaker to supply a 5HP 240V motor with R90 #8 copper wire rated at 45amps and the thermal overload set for 35amps.

[dieselgman]

SL parts are still available... but certainly not in plentiful supply these days.

dieselgman

[stevo42]

Thanks dieselgman for the good service. I should have gone for the metal replacement 'cold start" cups. Maybe after Christmas I'll get back to you and spring for the others. The plastic ones work fine but i have to use the pliers too pull the plunger out. Thats not a big deal as the tool box is handy. Those old ones will not come apart even after soaking in penetrating oil then appealing heat, maybe I'll try putting some air pressure to them.

[dieselgman]

Your nylon replacements will free up eventually with some lubrication and a little use.

The only real problem with them is their susceptibility to decay in direct sunlight and to melting under very high temperatures.

Lister quit supplying them in metal in the mid 60's I believe.

dieselgman

[stevo42]

It is nice to be able to use the cups as our power went out last evening and the SL fired right up using the cups. The only other issue is the throttle handle. the tooth that engages the notched quadrant is frozen in the handle, hopefully I 'll have better luck freeing that up.  Sunlight on the new cups wont be a problem, as the new engine room is windowless and inclosed.  Thanks again for the help.

[LowGear]

Hmmmmm,

20 amp breaker/fuse on 120 volts sure sounds like 2400 watts to me.  Why is the math different on a generator?

Casey

Do you want one leg running while the other is "off line"?