Oh Joy, another electric problem

A 50 amp circuit has 2 50amp breakers allowing the use of 100amps in your coach (50 amps on each leg) and yet you talk about a 40 amp breaker.
If we put that aside, nothing you do (assuming you have ensured that all things that might create a load are off) should cause a circuit to trip immediately as you describe unless the breaker itself is shot or you have a ground fault issue somewhere.
If the new 50 amp extension is not the issue (same issue old and new) then you need to see if you can get ANY cord from your coach to the plug without the current house cord in the loop or you need to turn off the main breaker in the RV to ensure house power cannot reach any RV circuit. If you do that and the breaker trips when you plug in then you have a bad cord. If it does not trip but does trip when you power up (again, all circuit breakers off) then you have a bad ground/neutral in the circuit and you need to get it fixed quickly!

jimo wrote:
the breaker trips when it is plugged in (immediately). What really amazes me is I have been using this same setup for the past 4 years and nothing bad has happened.

what happens when u plug it in with the MH breaker turned off??
the fact it blows instantly sounds like a dead short to ground in the TS or main MH power cord?

jimo wrote:
Pulled into my sister's driveway that has a 50 amp plug. We have camped there for many years during Christmas and everything has been fine.

key words - "many years" and "everything has been fine". *Circuit breakers get tired with age. Your sister might just need a new circuit breaker.

the breaker trips when it is plugged in (immediately). What really amazes me is I have been using this same setup for the past 4 years and nothing bad has happened.

IF you are using electric heat or A/C turn some off for a bit, You may need to turn two of them off.

Once your house batteries are full up,, Try again.

40A fuse? How old is the house electrical wiring?

Does the 40A house circuit have a GFI ground fault interupter (not possible with a fused circuit)? A little bit of moisture will trip a GFI.

If the fuse blows with 2 power cords not attached to anything I'd look at the house socket the cord plugs into.

Best,
- bob

Simple things first
1. Unplug coach from house, does the house breaker trip. If yes, it is house wiring. If no, it is coach wiring.
2. Flip to off all breakers in the coach, does the house beaker trip? If yes, it is something between the end of the cord and the coach breaker box. If no, then it is a circuit in the coach.
3. Flip on the coach main breaker but leave off other breakers, does the house breaker trip? If yes, it is the coach breaker box. If no, then it is a circuit in the coach.
4. Flip on and then off one coach circuit at a time until the house breaker trips. When it does, you have found the coach circuit that is causing the problem.
5. If the house breaker never trips in #4 above, but does trip when all the circuits are on, then I suspect you are overloading the 40 amp house circuit.

Sounds like you have a problem in your Transfer switch.
Shore power goes through the transfer switch before reaching your circuit breaker panel. Could also be a shorted line from transfer switch going out to shore power cord. As you say works fine on Generator so the problems not with your circuit breaker. I dought you have overloaded the 40 amp in your sisters house unless you have everything turned on. But than again no problem with gennie.

it trips "under load"

or it trips as soon as you plug in ?

which is it..

jimo wrote:
Pulled into my sister's driveway that has a 50 amp plug. We have camped there for many years during Christmas and everything has been fine. I have to use an 50 amp extension that attaches to our power cord on our Zephyr. Everything worked just fine for a couple of hours and then the power was gone. Tried again and the 40 amp fuse on their home kept breaking. It still is tripping. Purchased a new 50 amp extension cord and everything is still tripping. When unplugged the coach works just fine on the generator. My best guess is the short is in the power cord since when I turned off all of the switches in the coach the breaker kept tripping. Any of you pros out there have a suggestion?

Lets keep it simple. You have a 40 amp breaker supplying a 50 amp RV. 50 amp RV service is REALLY 2 50 amp breakers for a total of 100 amp available service. You have 60 amps LESS than the coach is designed for. EACH AC (or Heat Pump if you are in cold weather) will pull anywhere from 14 to 16 amps EACH. A Zephyr has 3 HP units. So, if you are running all 3 HP units, you are OVER the 40 amp for the circuit supply breaker by 2 amps. This does NOT take into account the Refer/the Inverter/charger/ or any other 120 appliances you may be running. If you have a Zephyr and have either a Furnace or Aquahot system (deopending on how old your RV is) run the furnace/ Aqua hot if you need heat. If you are in a warm climate and need AC, then you have no choice other than running just 2 of the 3 units. Doug