Is it a legal requirement to install a crash tested system in my vehicle? What laws govern what or I should carry water and my system in my vehicle?

Whilst there is not a "black and white" law stipulating that only a crash tested professionally fitted system should be fitted, there are some clear guidelines provided:

Firstly, authorities take overloaded vehicles very seriously. Whilst an over weight vehicle can be an issue practically - causing wear and tear on a vehicle, it also is a serious safety hazard. For this reason, checking your vehicle payload and axle weights and remaining within your specified payloads is a legal requirement.

For customers carrying a tank of water on a vehicle, calculating the correct size of system to carry is especially important, as the GVW (Gross Vehicle Weight) can quickly be exceeded with all the additional equipment and components normally required to be kept on board.

Once the correct size of system has been calculated based on the GVW, the weight of people the vehicle is capable of carrying, and the weight of all equipment, the next decision must be how to secure the system to the vehicle. This decision is not something to take lightly, because as the DoT (Department of Transport) say, securing a load within a vehicle should be of primary importance to all road users (regardless of if that's a loved one being secured by wearing a seat belt, or a lorry full of groceries being correctly packed).

The DoT have a code of practice on the safety of LCV's carrying loads which is fairly generic in nature based on the 1000s of variety of load carrying vehicles on the roads. However, Regulation 100 of the Road Vehicles (Construction and Use) Regulations 1986 as amended, requires that a vehicle should not be used or loaded in such a way that it endangers either those using the vehicle or other road user.
The DoT also mention within their load carrying guide that the centre of gravity of the equipment being carried should be a low as possible as well safety secured by adequate means to match the load being carried.
A water tank based system is classed as a dynamic force load. This means rather than the load being static, the water within a tank is moving and thus dynamic - thus meaning more velocity. WIth this in mind, more strength and force is required to secure a dynamic load than a static load.
The combined strength of the load restraint system must be sufficient to withstand a forwards force not less than the total weight of the load to prevent the load moving under severe braking, and half the weight of the load moving backwards and sideways. Even at low speeds, the forces acting on a load when the vehicle is moving can be high enough for the load to move.
Whilst as we mention there is no law that insists you must have a crash tested system, with the above information in mind, the following should also be considered:
You could be prosecuted for causing the death of an employee or a member of the public due to negligence if its found that the system is lacking in some form, and/p has not been professional installed and is the cause of the injury.
Negligence could be viewed as ignorance or the lack of effective processes, like failing to comply with existing guidance on vehicle load carrying and payloads. This can result in substantial fines for the company or individuals in the case of a partnership or sole trader.

In Summary:

Whilst none of us like to think of the worst case scenario, the potential for a 1000kg worth of volatile water and equipment to cause injury to yourself, an employee, or a member of the public is high.

So by having a professional crash tested system installed, you not only adhere to DoT standards and regulations, but more importantly, you have the peace of mind of knowing you have not cut corners. Risking injury, insurance invalidation, or possible prosecution is not worth it for the relative low cost of a professionally crash tested system.

Read more reasons here as to the benefits of a professionally installed system.