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Will a fused element dry fire?
Yes it can, but the resistor (fuse) reduces chance of dry fire

Dry fire
Energy Smart has dry fire protection
Dry Fire prevention is not 100% sure.
What is dry fire
Water heater must be full of water before electricity is turned ON.
If water is not covering elements, then upper element will dry-fire ... or instantly burn out.
Dry fire only happens when tank is empty of water.

Some water heater elements claim to have dry fire protection.
This will help prevent dry-fire incident, but not foolproof.

To prevent dry fire:
Fill tank completely with water, until water is coming out of bathtub spout, then open TP valve to release last of air from inside tank, and lastly, turn power ON.

Test elements if you suspect dry fire is causing water heater not to work:

Resources
How to test elements
Full troubleshoot for electric water heater
How electric water heaters work


Rheem marathon element wrench
Buy:
1-7/8" water heater element wrench/ Use screwdriver for handle
Professional 1-7/8" sockets

Buy socket:
1-7/8" socket with 3/4" drive
1-7/8 sockets at Amazon
24" breaker bar with 3/4" drive
Rheem marathon element


Upper element has a fuse to help prevent dry fire, but Marathon elements are interchangeable as long as length of element does not conflict with tank diameter. 
Use 1-7/8 element wrench
1.5" UNF straight fine thread not 1" NPSM straight// 12-14 threads per inch vrs 11.5
Read parts .pdf before buying
Buy:
SP610160/ 4500 watt copper/ Best value
SP213670/ 4500 watt

SP210197/ 3800 watt
SP210201/ 3000 watt
SP210198/ 3800 watt
SP213480/ 3000 watt
SP213540/ 2000 watt
SP610130/ 3800 watt
SP213820/ 3800 watt
SP610140/ 5500 watt
SP610150/ 3000 watt

Resources
Marathon parts sheet/ pdf
Regular water heater elements/ identify and replace
Why does marathon water heater have fused upper elements? 
Question: Do I really need fused upper element if I am smart enough to never dry fire the upper element?  
Seems to me its just another part to fail.
Dry fire
Answer 1:
Fused upper element is NOT required for tank to operate normally.
Fuse is not ordinary fuse that you can reset... it is a resistor that is added inside head of element.
Upper element dry fires.. but lower element rarely does.

What is dry fire?
Element must be covered with water when power is applied, or upper element will burn out.
Why only the upper element dry fires? Because 30 amp residential water heaters are non-simultaneous, which means upper element receives power first... so it is first to burn out. With non-simultaneous water heater, the lower part of tank does not receive power until upper element heats first... but if upper element dry fired, the top of tank never gets heated, so lower element never receives power... and tank is cold.
Resource
How electric water heaters work

Same answer different words:
Marathon upper elements are fused to help prevent dry fire. Not 100% prevention... just resistance to dry fire.
Elements will instantly burn out if they are not fully immersed in water.
Upper element will dry fire, but usually not the lower element.
Reason: Marathon is ordinary non simultaneous water heater: Upper element is ON, lower element is ON, or both elements are OFF
When tank is empty, the upper thermostat is cold.
Thermostats are mechanical bi-metal switches. [They require no electricity. They work with any AC voltage.]
When upper thermostat is cold, the bi-metal switch is positioned to send power to upper element.
When tank is cold, no power will flow to lower part of tank, until upper thermostat reaches set point.
When power is applied to cold tank, power flows to upper element. If tank is not full of water, then upper element dry fires. Lower element is unaffected.
When upper element is not working, then tank will not heat any water because lower element will never receives power unless upper part of tank is heated to set point.
If upper element is burned out with dry fire, the water heater heats no water.
Marathon elements are expensive by comparison to ordinary elements.
If you dry fire ordinary element, the replacement cost is $10-20 from local hardware store.
Marathon elements are not available at local hardware.
Resources:
How to wire thermostats
How thermostats work
He is thinking using the 105 marathon as the primary grid powered and the 50 gallon using a techluck preheat.    Im interested to see how the whole thing will work.



-Dave
    Where have you seen fused elements?

Resource
http://www.ilovewaterheaters.com/products/service-parts/sp610160/

All the consumer marathon come with standard fused upper elements.    Its possible to replace with titanium fused upper and I think I will get the titanium for the upper because of the way I use my water heater as an energy battery,  the upper does come on a lot.  Much more so than in a standard instantaneous grid install.
Resource
http://www.ilovewaterheaters.com/products/service-parts/sp213790/

The only question I have why dry firing a upper element is more prevalent than dry firing a lower. Is it because if the upper element fuse trips on dry fire the lower never gets power and therefore never gets dry fired?



    Dry fire is probably big problem at corporate:
1) Thanks for fused element.
I think these elements are dry fire 'resistant'
Some of the new electronic water heaters show dry fire elements... but with further reading, they say resistant.
Will keep my eye out for information about this

2) Upper element dry fires.. but lower element rarely does
Because most water heaters are non-simultaneous: the upper thermostat sends power to upper element first, and only after upper part of tank is heated does the upper thermostat switch and send power to lower part of tank
Resource
http://waterheatertimer.org/How-it-works.html

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