Isolated DC-DC first, then the coolant piping

Published 2026-10-26 · 7 min read · Beijing Hypart Technology

Two numbers govern the coolant loop of a hydrogen fuel cell system: 100 Ω/V and 500 Ω/V. They are often read as properties of the DC/DC converter. They are not. Both are insulation-resistance requirements derived from the current a person can survive, and between them they decide how much of the loop can be flexible hose and how much can be stainless steel tube.

Where the two numbers come from

IEC 60479-1 sets out the physiological effect of current through the human body. IEC 62282-3-100, the safety standard for stationary fuel cell power systems, adopts those limits as insulation-resistance rules:

• 100 Ω/V — a floating DC circuit with no AC coupling. A fault current of 10 mA is the accepted ceiling, so the required insulation resistance is the circuit voltage divided by 10 mA: 100 Ω per volt.
• 500 Ω/V — a circuit conductively tied to an AC grid or a high-voltage DC bus. Alternating current is more dangerous at the same magnitude, so the limit drops to 2 mA and the rule tightens to 500 Ω per volt.

Which rule applies follows from the system architecture. It does not follow from the brand of converter.

Coolant conductivity sets the floor

Liquid-cooled stacks run deionised water or a water-glycol mix, held at or below 5 µS/cm to limit leakage through the loop. At that conductivity the insulation resistance of the cooling circuit is set by three things:

• the length of insulating hose in the loop
• the bore of the pipe, which is the wetted volume of coolant
• the DC potential the loop sits at

Only the third is a decision about the converter. The first two are piping decisions, and they carry most of the cost.

Worked comparison: 500 V stack, 800 V bus

Take a stack with a 500 V open-circuit voltage feeding an 800 V DC bus — the common pairing in stationary plants.

The difference is a factor of five in mandatory insulation resistance. More useful than the ratio is what it does to the layout.

The parallel-stack problem

In a multi-stack plant the stacks share one coolant loop, so their insulation resistances sit electrically in parallel: n stacks give roughly 1/n of the resistance of one. A plant that meets 400 kΩ with a single stack needs four times that at four stacks, and past a certain point the only lever left is hose length.

Long hose is an expensive way to buy resistance. It adds pressure drop the pump has to pay for, it insulates the coolant from the radiator that is trying to cool it, and it ages. Galvanic isolation removes the reason for it: with each stack on its own insulation boundary, adding a stack adds a boundary instead of diluting one.

What that does to the piping

Once the loop no longer carries the insulation duty, its geometry can be chosen for flow and cleanliness instead:

• At the stack — a short silicone hose (−40 to +180 °C, electrically insulating) long enough to take up movement and isolate vibration.
• Distribution and long runs — 316L stainless tube and manifold assemblies: fixed bore, welded ports on the pitch the rack needs, electropolished, hydro-tested at 16 bar for three minutes and helium leak-tested.
• Upstream of both — the isolated DC/DC converter that makes the short-hose layout possible in the first place.

Bore follows stack power and coolant flow: small-bore tube for a single stack, large manifold bodies where several stacks are paralleled and flow has to be balanced across them.

The same loop in a data centre

Cold-plate loops in AI and HPC halls run to the same coolant spec — below 5 µS/cm — and meet the same problem at larger scale: hundreds of parallel drops, each one a leak path and a pressure-drop penalty. Rigid 316L manifolds with welded ports on a fixed pitch, plus a short flexible jump to each cold plate, keeps both under control. The insulation reasoning differs, because the CDU is not a fuel cell, but the pipe architecture is the same — which is why one tube line serves both markets.

Ordering the three parts together

Insulation requirement, hose length and tube layout are one decision, not three. Send the stack voltage window, the bus voltage and the number of stacks in parallel. The insulation rule follows from those three numbers, and so does the split between hose and tube.

References
IEC 60479-1 — Effects of current on human beings and livestock.
IEC 62282-3-100 — Stationary fuel cell power systems, safety.