<p>I'm building a 40x40x40cm corexy and I am quite impatient so I want the heated to reach the target temperature as fast as possible, so I ordered a <a href="https://www.aliexpress.com/item/400X400mm-1200W-220V-w-NTC-100K-Thermistor-Keenovo-Silicone-Heater-Pad-for-Huge-Mega-Cube-3D/32550597606.html?spm=a2g0s.9042311.0.0.Io1mMV" rel="noreferrer">Keenovo silicone heater</a>
It is a 220VAC 1200Watt bed, so I really want to make sure that it is safe to use.
I also bought a <a href="http://www.crydom.com/en/products/catalog/series-1-240-ac-panel-mount.pdf" rel="noreferrer">Crydom D2450</a> SSR.</p>

<p>Could someone tell me if the wiring in the diagram I made below is safe?</p>

<p>Do I need to put a fuse or some other kind of safety?</p>

<p><a href="https://i.stack.imgur.com/dd7TA.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/dd7TA.jpg" alt="wiring"></a></p>

The answer is, yes you can, but you need to follow guidelines.
Reversing +/- or otherwise incorrectly connecting power can destroy your electronics and cause fire hazard.
From
RepRap wiki - RAMPS 1.4
Maximum Input Voltage Power Supply without diode There are three
limiting factors to the maximum voltage that you can put into the
RAMPS:
The Arduino Mega maximum input voltage Filtering capacitor maximum
voltages PTC fuse maximum voltages First, the 1N4004 diode connects
the RAMPS input voltage to the Arduino Mega which has a recommended
maximum input voltage of 12 volts. If your board does not have this
diode soldered in (or if you cut it), you will need to power the Mega
through the USB connector or through a separate 5v line, but this
allows a higher RAMPS voltage.
Second, most boards use 25v or 35v aluminum electrolytic capactors
(C2, C3, C4, C6, C7, C9, and C10). To be safe, you should only go to
half of your rated maximum voltage -- thus if your board has 35v
capacitors (code VZA) then you should use a maximum input of 17.5v.
The absolute maximum voltage is determined by the pololu servo
drivers, which themselves are limited to 35V.
Third, the MF-R500 (5A) PTC fuse is rated to 30V and the MF-R1100
(11A) PTC fuse is rated to 16V. They will need to be replaced with
real fuses.
Power Supply with diode If your board has a 1N4004 diode soldered in,
do not apply more than 12 V to it. Original flavor Arduino Mega are
rated to 12 V input. While Arduino Mega 2560 can take 20 V, it is not
recommended.