If you’re looking for dividend yields that are both high and reliable, it’s tempting to focus on the FTSE’s biggest dividend stocks.

What’s unusual about the current market is that the biggest dividend stock of them all — Royal Dutch Shell (LSE: RDSB) — is currently offering a dividend yield of 7.4%.

Shell is famous for not having cut its dividend since World War II. Chief executive Ben van Beurden is keen to maintain this record, which attracts a lot of long-term shareholders. But a yield this high is very often a sign of a dividend that’s unaffordable.

Is a cut likely?

With profits recovering from the oil crash, Shell trades on a 2016 forecast P/E of 23, falling to a P/E of 12.5 for 2017. These figures look reasonable to me.

While the group’s net debt of $75bn is higher than I’d like to see, Shell’s low borrowing costs and long-term outlook should mean that debt-related problems are unlikely. However, this rising tide of debt could put pressure on dividend payments.

Shell’s dividend wasn’t covered by earnings last year, and isn’t expected to be covered this year. If earnings don’t recover next year, I believe the chances of a cut could rise sharply.

Luckily, Shell’s earnings are expected to rise to $2.05 per share in 2017. This would give dividend cover of 1.1 times. Free cash flow should also improve, assuming oil manages to climb above $50. In this scenario, I think a dividend cut is unlikely.

However, there’s a risk that oil will stay low. Shell may reach a point where borrowing money to pay the dividend no longer makes sense. If we use this year’s forecast earnings of $1.11 per share as a baseline, I estimate that in a worst-case scenario, the dividend could be cut by 45% to $1 per share.

Doing this would reduce Shell’s dividend yield to 4%. I don’t think such a big cut is likely, but the reality is that a yield of 4% would still be attractive. Indeed, some investors would argue that it would be more attractive because it would be affordable!

A better alternative?

While I rate Shell as an income buy, the risk of a dividend cut means that for me, it’s not a best buy.

If you’re look for a high dividend yield that can keep pace with inflation, then utility group SSE (LSE: SSE) might be a better alternative. SSE shares offer a 6% yield and have risen by 24% over the last 10 years, compared to just 13% for the FTSE 100.

SSE’s policy is to increase its dividend in line with RPI inflation. This policy is expected to be maintained until at least the 2018/19 financial year.

Last year’s dividend cover of 1.34 times suggests to me that SSE’s payout should remain affordable, especially as the group believes there’s now “increased clarity” on future energy policy.

City analysts are also taking a more positive view of this stock. Forecast earnings for the current year have risen since March, recouping the losses seen last year and putting the shares on a forecast P/E of 13.

I’d be happy to add to my own holding at current levels, and rate the shares as an income buy.

Read this before you act

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Roland Head owns shares of Royal Dutch Shell and SSE. The Motley Fool UK has recommended Royal Dutch Shell B. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.